gallium: New PIPE_BUFFER_USAGE_FLUSH_EXPLICIT flag for buffer_flush_mapped_range.

gallium: New PIPE_BUFFER_USAGE_FLUSH_EXPLICIT flag for buffer_flush_mapped_range.

When a buffer was mapped for write and no explicit flush range was provided
the existing semantics were that the whole buffer would be flushed, mostly
for backwards compatability with non map-buffer-range aware code.

However if the buffer was mapped/unmapped with nothing really written --
something that often happens with the vbo -- we were unnecessarily assuming
that the whole buffer was written.

The new PIPE_BUFFER_USAGE_FLUSH_EXPLICIT flag (based from ARB_map_buffer_range
's GL_MAP_FLUSH_EXPLICIT_BIT flag) allows to clearly distinguish the
legacy usage from the nothing written usage.