Fixes surface alpha

[JujuAdams]

When drawing ImGui_GM over other content, the alpha channel of the internal ImGui_GM surface regularly allows background images to be seen through the surface even though such images should be obscured. This is a common issue with any alpha blended content on GameMaker surfaces and is not limited to ImGui_GM.

This PR patches the problem by implementing premultiplied alpha for ImGui_GM rendering. This boils down to a different blend mode in .__Render() and an alpha multiplication in the fragment shader of shdImGui.