gl: pass alpha value to shader

In gaussian filter, if the source is opaque, we should pickup
sampler color of (0, 0, 0, 1) instead of (0, 0, 0, 0).  However, since
gaussian blur is done on a RGBA texture, we don't know whether the original
source is opaque or not.  Instead, we pass the alpha value in uniform.
xcb: set correct extends on filtered picture