mesa: support multisample textures in framebuffer completeness check

mesa: support multisample textures in framebuffer completeness check

- sample count must be the same on all attachments
- fixedsamplepositions must be the same on all attachments
(renderbuffers have fixedsamplepositions=true implicitly; only
multisample textures can choose to have it false)

V2: - fix wrapping to 80 columns, debug message, fix for state moving
      from texobj to image.
    - stencil texturing tweaks tidied up and folded in here.

V3: - Removed silly stencil hacks entirely; the extension doesn't
      actually make stencil-only textures legal at all.
    - Moved sample count / fixed sample locations checks into
      existing attachment-type-specific blocks, as suggested by Eric

V4: - Removed stencil hacks which were missed in V3 (thanks Eric)
    - Don't move the declaration of texImg; only required pre-V3.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
[V2] Reviewed-by: Paul Berry <stereotype441@gmail.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>