Fixes for CVE-2014-0250

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.
Set checks to be strict and also check xorBpp field

This patch:
* renames bpp to xorBpp ;
* changes checks to strict ;
* adds checks on the xorBpp field
Check for bpp > 0

Bpp == 0 just makes no sense
Check that bpp has reasonable value

As bpp is often used for malloc computations, let's check that it has
a reasonable value.
Merge pull request #1874 from hardening/CVE-2014-0250

Fixes for CVE-2014-0250