Fix for pages being repeated when running on Android Honeycomb.

Fixes bug #695191 - Mupdf Build49/armv7a & Android 3.1: cycles
through subset of pages & page scrubber

The problem here was that in Honeycomb, various bitmap operations
(including drawing via JNI) do not update the bitmap generation count.
When hardware acceleration is enabled, this means that the underlying GL
layer is not aware that the bitmap has changed, and ends up reusing old
textures.

To workaround this, we erase the bitmap before drawing the page. Erase
appears to be the only operation I could find (after pouring through the
source), which actually increments the generation count. The other option
would have been to disable hardware acceleration, but that was far less ideal.