i965/msaa: Treat GL_SAMPLES=1 as equivalent to GL_SAMPLES=0.

i965/msaa: Treat GL_SAMPLES=1 as equivalent to GL_SAMPLES=0.

EXT_framebuffer_multisample is a required subpart of
ARB_framebuffer_object, which means that we must support it even on
platforms that don't support MSAA.  Fortunately
EXT_framebuffer_multisample allows for this by allowing GL_MAX_SAMPLES
to be set to 1.

This leads to a tricky quirk in the GL spec: since
GlRenderbufferStorageMultisamples() accepts any value for its
"samples" parameter up to and including GL_MAX_SAMPLES, that means
that on platforms that don't support MSAA, GL_SAMPLES is allowed to be
set to either 0 or 1.  On platforms that do support MSAA, GL_SAMPLES=1
is not used; 0 means no MSAA, and 2 or higher means MSAA.

In other words, GL_SAMPLES needs to be interpreted as follows:
  =0  no MSAA (possible on all platforms)
  =1  no MSAA (only possible on platforms where MSAA unsupported)
  >1  MSAA (only possible on platforms where MSAA supported)

This patch modifies all MSAA-related code to choose between
multisampling and single-sampling based on the condition (GL_SAMPLES >
1) instead of (GL_SAMPLES > 0) so that GL_SAMPLES=1 will be treated as
"no MSAA".

Note that since GL_SAMPLES=1 implies GL_SAMPLE_BUFFERS=1, we can no
longer use GL_SAMPLE_BUFFERS to distinguish between MSAA and non-MSAA
rendering.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>