driver: various fixes for mesa integration

- fix crash on multiple non-consecutive vertex elements
- don't touch alpha factors when alpha blending disabled, to prevent
  warning flood
- add silent flag to translate functions, to make probing in
  etna_screen emit no warnings
- add missing pipe caps
- force NPO2 textures to true, as this is required by GLES2
  this will likely cause problems on on GPUs that don't really
  have support for this, we'd need to build a workaround
- temporary flush front buffer hack for mesa fbdev support