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/GPU_immediate.h

@@ -47,6 +47,7 @@ void immBindBuiltinProgram(GPUBuiltinShader shader_id);
  * Extend immUniformColor to take Blender's themes
  */
 void immUniformThemeColor(int color_id);
+void immUniformThemeColor3(int color_id);
 void immUniformThemeColorShade(int color_id, int offset);
 void immUniformThemeColorShadeAlpha(int color_id, int color_offset, int alpha_offset);
 void immUniformThemeColorBlendShade(int color_id1, int color_id2, float fac, int offset);