mesa: handle GL_FRONT after translating to it
Without this, we end up throwing errors on code along these lines when rendering using single-buffering:
GLint att;
glGetIntegerv(GL_READ_BUFFER, &att);
glGetFramebufferAttachmentParameteriv(GL_READ_FRAMEBUFFER, att, ...);
This is because we internally translate GL_BACK (which is what glGetIntegerv returned) to GL_FRONT, which we don't handle in the Desktop GL case. So let's start handling it.
This fixes the GLTF-GL33.gtf21.GL2FixedTests.buffer_color.blend_color test for me.
Fixes: e6ca6e58 ("mesa: Handle pbuffers in desktop GL framebuffer attachment queries")