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.

Solve this by passing through the values as integers, and masking off the upper 16 bits in the vertex shader (which could be 1 due to sign extension).