Everywhere it's used a regular checkGLcall will do just as well.

This allows us to drop the load time conversion and the clear
readback hack and replaces it with a color fixup in the fixed
function pipeline replacement.

Based on a patch by Stefan Dösinger. This is more flexible, and allows
the shader backend implementation to be simpler, since it doesn't have
to know about specific formats. The next patch makes use of this.

Also remove the silly typedef.

Allows double indirection MS ShellDlg -> MS UI Gothic linked to system
gothic to work as expected.

Note that constants_set was never actually NULL for
shader_glsl_load_constantsB() and shader_glsl_load_constantsI().