I can't find any performance benefit to using the GL path and the code
renders this trapezoid incorrectly:

  		                 top: FIXED   29.50
		              bottom: FIXED   30.00
		            left top: POINT    0.00,   29.50
		         left bottom: POINT    0.00,   30.50
		           right top: POINT -127.50,   29.50
		        right bottom: POINT   52.50,   30.00

This should render a solid line from 0,30 to 52,30 but draws nothing.

The code also uses an area computation for trapezoid coverage which
does not conform to the Render specification which requires a specific
point sampling technique.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>