intel: Don't keep intel->pClipRects, and instead just calculate it when needed.

intel: Don't keep intel->pClipRects, and instead just calculate it when needed.

This avoids issues with dereferencing stale cliprects around intel_draw_buffer
time.  Additionally, take advantage of cliprects staying constant for FBOs and
DRI2, and emit cliprects in the batchbuffer instead of having to flush batch
each time they change.