mesa/st: Fix user buffer size computation when stride is zero.

mesa/st: Fix user buffer size computation when stride is zero.

Although for GL a zero stride means tightly packed elements, Mesa
internally uses zero strides for constant arrays.

Therefore user buffers need to be defined from

  buffer_offset + src_offset + min_index*stride

to

  buffer_offset + src_offset + max_index*stride + elem_size

Simplifying the later with (max_index + 1)*stride will give zero
sized buffers.

This change also aggregates the st_context's info about user buffers
into a single array.