Moved inverse to standard_colors.cpp

This is the only place it's used, and it's no longer an SDL-related util now that we have color_t. It could
potentially become part of color_t, but I'd have to think about it.

This also removes the sdl/utils.hpp dependency.

src/font/standard_colors.cpp

@@ -14,10 +14,19 @@
 
 #include "font/standard_colors.hpp"
 
-#include "sdl/utils.hpp"
-
 namespace font {
 
+// TODO: evalulate whether this should become a color_t member
+static color_t inverse(const color_t& color)
+{
+	return {
+		static_cast<uint8_t>(255 - color.r),
+		static_cast<uint8_t>(255 - color.g),
+		static_cast<uint8_t>(255 - color.b),
+		255 // TODO: should we respect alpha?
+	};
+}
+
 const color_t
 	NORMAL_COLOR    {0xDD, 0xDD, 0xDD},
 	GRAY_COLOR      {0x77, 0x77, 0x77},

src/sdl/utils.cpp

@@ -2462,15 +2462,6 @@ SDL_Rect get_non_transparent_portion(const surface &surf)
 	return res;
 }
 
-color_t inverse(const color_t& color) {
-	return {
-		static_cast<Uint8>(255 - color.r),
-		static_cast<Uint8>(255 - color.g),
-		static_cast<Uint8>(255 - color.b),
-		0 // TODO: ehh?
-	};
-}
-
 surface_restorer::surface_restorer() : target_(nullptr), rect_(sdl::empty_rect), surface_(nullptr)
 {
 }

src/sdl/utils.hpp

@@ -409,8 +409,6 @@ void blit_surface(const surface& src,
 
 SDL_Rect get_non_transparent_portion(const surface &surf);
 
-color_t inverse(const color_t& color);
-
 /**
  * Helper class for pinning SDL surfaces into memory.
  * @note This class should be used only with neutral surfaces, so that