llvmpipe: clamp maxx,maxy to framebuffer size (in terms of tiles)

llvmpipe: clamp maxx,maxy to framebuffer size (in terms of tiles)

In some corner cases the right-most / bottom-most vertex can be
right on the edge of the framebuffer.  Because the maxx, maxy vals
are computed with a series of float/int, pixel/tile transformations
we can end up with maxx >= scene->x_tiles or maxy >= scene->y_tiles.
This leads to putting data into bins that never get processed, or
reset.  This becomes stale data that can lead to segfaults.

Clamping fixes this.