mesa: increase max texture image units and GLSL samplers to 16

mesa: increase max texture image units and GLSL samplers to 16

The max texture coord units is still 8.  All the fixed-function paths are
still limited to 8 too.  But GLSL shaders can use more samplers now.

Note that some texcoord-related data structures are declared to be 16
elements in size rather than 8.  This just simplifies the code in a few
places; the extra elements aren't accessible to the user.

These changes haven't been extensively tested yet, but sanity checking has
been done.

It should be possible to increase the max image units/samplers to 32 without
doing anything special.  Beyond that we'll need longer bitfields in a few
places.