OpenGL3 renderer supports transparent boot splash

[scgm0]

A little strange why opacity was forced before? The gl_compatibility renderer now supports a startup background color with transparency like other renderers.
Project settings:

forward+:

master gl_compatibility:

this pr gl_compatibility:

[clayjohn]

Makes sense! This code comes from before we had window transparency in Godot4. So it was just missed

[akien-mga]

Thanks!