Correctly set, use and update clipping regions of DCs

Intended commit message:

[WIN32K]

• Correctly use the Clip, Meta, Rao, API and Vis Regions in DCs
• Update the DC clipping object on demand each time a blt is performed
• Reduce the number of region allocated with handles when it's not needed

The goal is to improve clipping which is quite suboptimal in ReactOS at the moment (windows drawing over each other, etc...) and, incidentally, performance.