mesa: fix glGetFramebufferAttachmentParameteriv w/ on-demand FRONT_BACK alloc
mesa: fix glGetFramebufferAttachmentParameteriv w/ on-demand FRONT_BACK alloc

This fixes 66 CTS tests on st/mesa.

Cc: 12.0 <59f39c0db42d4479a46b02d4d2bc11120e37bb44@lists.freedesktop.org>
Reviewed-by: Nicolai Hähnle <0d0be6cf5ad662651be2dc5dfd44aa2b09d4c1cc@amd.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler