Fix T50976: Blender UI problems with certain theme files.

Core of the issue was that some of our Theme colors are RGB-only, but
were loaded as RGBA.

Note that tracking all possible cases is pretty impossible, so we'll
have to tackle those as they get reported am afraid.

source/blender/gpu/intern/gpu_immediate.c

@@ -45,6 +45,13 @@ void immUniformThemeColor(int color_id)
 	immUniformColor4fv(color);
 }
 
+void immUniformThemeColor3(int color_id)
+{
+	float color[3];
+	UI_GetThemeColor3fv(color_id, color);
+	immUniformColor3fv(color);
+}
+
 void immUniformThemeColorShade(int color_id, int offset)
 {
 	float color[4];