glamor: Fix glamor_poly_fill_rect_gl xRectangle::width/height handling

(Using GLSL 1.30 or newer)

The width/height members of xRectangle are unsigned, but they were
being interpreted as signed when converting to floating point for the
vertex shader, producing incorrect drawing for values > 32767.

v2:
* Use separate GL_UNSIGNED_SHORT vertex attribute for width/height.
  (Eric Anholt)

Reviewed-by: Eric Anholt <eric@anholt.net>
4 jobs for glamor_poly_fill_rect_gl-width-height-unsigned in 3 minutes and 48 seconds (queued for 1 second)