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.
José Fonseca
0c501bacef