Fallback to pixman when trapezoid mask is big.

Fallback to pixman when trapezoid mask is big.

 The trapezoid generating speed of the shader is relatively
 slower when the trapezoid area is big. We fallback when
 the trapezoid's width and height is bigger enough.
 The big traps number will also slow down the render because
 of the VBO size. We fallback if ntrap > 256

Signed-off-by: Junyan He <junyan.he@linux.intel.com>
Reviewed-By: Zhigang Gong <zhigang.gong@linux.intel.com>