[glsl-out] Rename identifiers containing two consecutive underscores since they are reserved

[Teodor Tanasoaia]

In addition, all identifiers containing two consecutive underscores (__) are reserved for use by underlying software layers. Defining such a name in a shader does not itself result in an error, but may result in unintended behaviors that stem from having multiple definitions of the same name.

from https://registry.khronos.org/OpenGL/specs/gl/GLSLangSpec.4.60.html#keywords