BACKEND_OPENGL_VERSION_GL tested but header not included

For context, on line 38, it checks for `#if defined(BACKEND_OPENGL_VERSION_GL)` to determine if we're dealing with OpenGL or GLES. However, BACKEND_OPENGL_VERSION_GL is only defined in gl_headers.h. Without including this header, this define is never set and, therefore, this class will always assume it's only for GLES.

_Originally posted by @haroonq in https://github.com/google/filament/issues/9724#issuecomment-3930412838_