Fonts: actually got light weight variant font support working

This replaces the broken implementation I added in fb6c85e70b.
See documentation in data/hardwired/fonts.cfg for explanation as to why it didn't work and
why this does.

Currently "light" is set to Lato Light, which is weight 300. Regular is weight 400.

data/hardwired/fonts.cfg

@@ -1,15 +1,38 @@
 #textdomain wesnoth
 
 [fonts]
-    # This is marked as translatable, to allow translators to
-    # provide different font orders: one just has to install the
-    # missing fonts without needing to change the gmo files
-    order=_ "Lato-Regular.ttf,DejaVuSans.ttf,Andagii.ttf,DroidSansJapanese.ttf,DroidSansFallbackFull.ttf,Junicode-Regular.ttf"
-    family_order=_ "Lato"
+    # NOTE: font order keys are marked as translatable to allow translators to
+    # provide different font orders: one just has to install the missing fonts
+    # or add them to wesnoth/fonts.
 
-    # Used by GUI2 only, hence no [font] blocks for these. The font files are
-    # also automatically determined.
-    family_order_monospace=_ "DejaVu Sans Mono"
+    #
+    # Pango text-related keys.
+    #
+    # All respective font files are automatically loaded.
+    #
+    # NOTE: these are technically supposed to be lists to provide fallbacks
+    # like the SDL_TTF code does. However, there's a weird bug in Pango itself
+    # where a comma seperated list of fonts will result in newlines not being
+    # rendered and font space metrics being off on Windows.
+    #
+    # Additionally, despite what one might assume, "Lato" doesn't appear to
+    # load all Lato font variants (it has a bunch of different weight ones).
+    # I had thought one could specify different weights and Pango would
+    # automatically use the variant for that weight, but no dice. Instead, we
+    # need to set the weight variant we want as the distired font familt at
+    # render time. Right now we only have use of a "light" variant, but others
+    # could be added at some point.
+    #
+    # -- vultraz, 2018-02-14
+    #
+    family_order= _ "Lato"
+    family_order_monospace= _ "DejaVu Sans Mono"
+    family_order_light= _ "Lato Light"
+
+    #
+    # Legagy SDL_TTF stuff.
+    #
+    order= _ "Lato-Regular.ttf,DejaVuSans.ttf,Andagii.ttf,DroidSansJapanese.ttf,DroidSansFallbackFull.ttf,Junicode-Regular.ttf"
 
     [font]
         name="Lato-Regular.ttf"

src/font/font_config.cpp

@@ -99,6 +99,7 @@ static bool add_font_to_fontlist(const config &fonts_config,
 
 t_string family_order_sans;
 t_string family_order_mono;
+t_string family_order_light;
 
 /***
  * Public interface
@@ -141,12 +142,18 @@ bool load_font_config()
 
 	family_order_sans = fonts_config["family_order"];
 	family_order_mono = fonts_config["family_order_monospace"];
+	family_order_light = fonts_config["family_order_light"];
 
 	if(family_order_mono.empty()) {
 		ERR_FT << "No monospace font family order defined, falling back to sans serif order\n";
 		family_order_mono = family_order_sans;
 	}
 
+	if(family_order_light.empty()) {
+		ERR_FT << "No light font family order defined, falling back to sans serif order\n";
+		family_order_light = family_order_sans;
+	}
+
 	std::vector<font::subset_descriptor> fontlist;
 
 	for(auto font : utils::split(fonts_config["order"])) {
@@ -170,6 +177,8 @@ const t_string& get_font_families(family_class fclass)
 	switch(fclass) {
 	case FONT_MONOSPACE:
 		return family_order_mono;
+	case FONT_LIGHT:
+		return family_order_light;
 	default:
 		return family_order_sans;
 	}

src/font/font_options.hpp

@@ -25,13 +25,16 @@ namespace font
 enum family_class
 {
 	FONT_SANS_SERIF,
-	FONT_MONOSPACE
+	FONT_MONOSPACE,
+	FONT_LIGHT
 };
 
 inline family_class str_to_family_class(const std::string& str)
 {
 	if(str == "monospace") {
 		return FONT_MONOSPACE;
+	} else if(str == "light") {
+		return FONT_LIGHT;
 	}
 
 	return FONT_SANS_SERIF;

src/font/pango/font.hpp

@@ -36,9 +36,6 @@ public:
 			if(style & pango_text::STYLE_BOLD) {
 				pango_font_description_set_weight(font_, PANGO_WEIGHT_BOLD);
 			}
-			if(style & pango_text::STYLE_LIGHT) {
-				pango_font_description_set_weight(font_, PANGO_WEIGHT_LIGHT);
-			}
 			if(style & pango_text::STYLE_UNDERLINE) {
 				/* Do nothing here, underline is a property of the layout. */
 			}

src/font/text.hpp

@@ -140,7 +140,6 @@ public:
 		STYLE_BOLD = 1,
 		STYLE_ITALIC = 2,
 		STYLE_UNDERLINE = 4,
-		STYLE_LIGHT = 8,
 	};
 
 	/***** ***** ***** ***** Query details ***** ***** ***** *****/

src/gui/widgets/helper.cpp

@@ -41,7 +41,6 @@ font::pango_text::FONT_STYLE decode_font_style(const std::string& style)
 		{"bold",      font::pango_text::STYLE_BOLD},
 		{"italic",    font::pango_text::STYLE_ITALIC},
 		{"underline", font::pango_text::STYLE_UNDERLINE},
-		{"light",     font::pango_text::STYLE_LIGHT},
 	};
 
 	if(style.empty()) {