Fix "End Turn" button placement on low-res themes.

Fixes #6544.

I am not sure how this was even remotely working as it was.

(cherry picked from commit 91fd53ac45)

data/themes/default.cfg

@@ -514,6 +514,19 @@
 
         {MINIMAP_THEME_600}
 
+        [change]
+            id=screen
+            rect="0,0,1024,600"
+        [/change]
+        [change]
+            id=sidebar-panel
+            rect="+0,0,1024,600"
+        [/change]
+        [change]
+            id=main-map
+            rect="=,+0,+842,600"
+        [/change]
+
         [change]
             id=label-hp
             font_size={DEFAULT_FONT_REALLYTINY}
@@ -615,6 +628,14 @@
         width=800
         height=600
 
+        [change]
+            id=screen
+            rect="0,0,800,600"
+        [/change]
+        [change]
+            id=sidebar-panel
+            rect="+0,0,800,600"
+        [/change]
         [change]
             id=main-map
             rect="=,+0,+618,600"