mesa: gl_NumSamples should always be at least one
mesa: gl_NumSamples should always be at least one

From ARB_sample_shading:

    "gl_NumSamples is the total number of samples in the framebuffer,
     or one if rendering to a non-multisample framebuffer"

So make sure to always pass in at least 1.

Signed-off-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Reviewed-by: Edward O`Callaghan <117173807d5a49976a891944633465c0daea7f98@alterapraxis.com>
Reviewed-by: Neil Roberts <822ec91a7ef82248fd36162ca8ac3283eba70848@linux.intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler