This patch introduce misc checks when receiving pointer updates. We check
that the cursor are in the bounds defined by the spec. We also check that
the announced mask sizes are what they should be.

This patch:
* renames bpp to xorBpp ;
* changes checks to strict ;
* adds checks on the xorBpp field

Bpp == 0 just makes no sense

As bpp is often used for malloc computations, let's check that it has
a reasonable value.