mesa: GL_EXT_texture_norm16 extension plumbing

mesa: GL_EXT_texture_norm16 extension plumbing

Patch enables use of short and unsigned short data for texture uploads,
rendering and reading of framebuffers within the restrictions specified
in GL_EXT_texture_norm16 spec.

Patch also enables those 16bit format layout qualifiers listed in
GL_NV_image_formats that depend on EXT_texture_norm16.

v2: expose extension with dummy_true
    fix layout qualifier map changes (Ilia Mirkin)

v3: use _mesa_has_EXT_texture_norm16, other fixes
    and cleanup (Ilia Mirkin)

v4: fix rest of the issues found

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Acked-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>