Evas fitlers: Implement SLOW font draw to alpha targets

Evas fitlers: Implement SLOW font draw to alpha targets

Well, raster did some great job at optimizing font draw... but only
to RGBA32 targets. In this font effects case, we also want to render
text on ALPHA buffers.

For now, reuse the existing alpha blending & glyph decompress
functions. It's MUCH easier, and works. Definitely slower than
decompressing on-the-fly and optimizing everything. But for now,
this will not even be the performance bottleneck in an effect
(blur will be a lot slower).