ClipToPicture test: add even more tests

* Allow scrolling the view,
* Add tests for ScaleBy and SetOrigin
* Add test for multiple clipping pictures using PushState.
BView: Allow resetting ClipToPicture

Just like for regions, ClipToPicture(NULL) removes any picture clipping
at the current state level (clipping in pushed states are not undone).
Add Alpha Masking support in ClipToPicture

Use AGG to implement ClipToPicture in a faster and better way.
There are things missing in this initial implementation:
* No support for PushState/PopState saving and restoring the picture.
* No support for nested clipping through PushState
* The clipping doesn't happen where you expect it when using SetScale()
* There are artifacts when scrolling and resizing clipped views
* The implementation uses more memory than it needs, as the clipping
bitmap is stored as RGBA32, yet only the alpha channel is used
* The clipping bitmap is rendered more times than it needs to. We need
some caching here.
Add new classes required for picture clipping

* agg_clipped_alpha_mask is a modified version of agg alpha mask class,
allowing us to offset the mask bitmap to follow the view position, and
also allows "inverse" clipping, where everything outside the bitmap is
considered inside the clipping region.
* AlphaMask is a container class keeping the ServerPicture, it's bitmap
rendering, and other relevant information. It will be used to save and
restore the clipping picture as part of the view state.
* Because these classes introduce more coupling within app_server, it's
not possible anymore to split out tst_app_server from
libtestappserver.so. Instead, move everything to libtestappserver.so,
except the things that actually need to not be there (to avoid
interferences with the host API). As a result, the previously introduced
stub cpp file to work around this problem isn't needed anymore.

The design for all this (and the previous commit) is Stippi's work.
Thanks for the advice and implementation hints!