Use shader to generate the temp trapezoid mask

Use shader to generate the temp trapezoid mask

 The old manner of trapezoid render uses pixman to
 generate a mask pixmap and upload it to the GPU.
 This effect the performance. We now use shader to
 generate the temp trapezoid mask to avoid the
 uploading of this pixmap.
 We implement a anti-alias manner in the shader
 according to pixman, which will caculate the area
 inside the trapezoid dividing total area for every
 pixel and assign it to the alpha value of that pixel.
 The pixman use a int-to-fix manner to approximate but
 the shader use float, so the result may have some
 difference.
 Because the array in the shader has optimization problem,
 we need to emit the vertex of every trapezoid every
 time, which will effect the performance a lot. Need to
 improve it.

Signed-off-by: Junyan He <junyan.he@linux.intel.com>