evas filters: Fix blur logic and GL buffer handling

evas filters: Fix blur logic and GL buffer handling

This corrects two things:
- the blur filter high-level logic, that lead to reusing some
  temporary buffers which contained garbage;
- the versatile gl buffer implementation so that it now properly
  switches between the RGBA_Image and the FBO content (yes, this
  is insanely slow and inefficient... but it works and that was
  the only point).