Set world framebuffer size based on minimum zoom.

This avoids reallocating buffers each time the player changes zoom level.

OpenRA.Game/Graphics/UISpriteRenderable.cs

@@ -32,6 +32,12 @@ namespace OpenRA.Graphics
 			this.palette = palette;
 			this.scale = scale;
 			this.alpha = alpha;
+
+			// PERF: Remove useless palette assignments for RGBA sprites
+			// HACK: This is working around the fact that palettes are defined on traits rather than sequences
+			// and can be removed once this has been fixed
+			if (sprite.Channel == TextureChannel.RGBA && !(palette?.HasColorShift ?? false))
+				this.palette = null;
 		}
 
 		// Does not exist in the world, so a world positions don't make sense