Make Color use uint for ARGB.

This is a more natural representation than int that allows removal of casts in many places that require uint. Additionally, we can change the internal representation from long to uint, making the Color struct smaller. Since arrays of colors are common, this can save on memory.

OpenRA.Mods.Common/Widgets/ColorMixerWidget.cs

@@ -68,7 +68,7 @@ namespace OpenRA.Mods.Common.Widgets
 				// Generate palette in HSV
 				fixed (byte* cc = &buffer[0])
 				{
-					var c = (int*)cc;
+					var c = (uint*)cc;
 					for (var v = 0; v < 256; v++)
 					{
 						for (var s = 0; s < 256; s++)