mesa: Add fall-back formats for unsupported snorm formats

This is always the way with real hardware and desktop OpenGL.  Some
hardware can't do some formats natively.  The alpha-only, luminance,
and intensity formats are usually the most problematic.  Some sized
formats can also be problematic.  This patch provides fall-back
formats for those that are not natively supported.

At some point it would be interesting to try providing
device-independent conversions using EXT_texture_swizzle.  The drivers
that support EXT_texture_swizzle could, for example, see
GL_LUMINANCE16_SNORM as MESA_FORMAT_SIGNED_R16 with a { r, r, r, 1 }
swizzle.  Care would need to be taken to prevent issues with using
those textures for FBO rendering.

This is the rest of the fix for glean's pixelFormats test on i965.

Reviewed-by: Marek Olšák <[email protected]>
Reviewed-by: Kenneth Graunke <[email protected]>

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