The wrong context was active when shaders were deleted.
This also fix the crash when switching scaling mode or anything that
deletes a KisOpenGLCanvas2.

Changed the KisOpenGL API that deal with contexts a bit, now is possible
to choose between switching on the shared context or a widget one.

To avoid overhead switching on a context that is already current, i've
added a check to see if the context we are trying to switch to is already current
or not (the check doesn't use any gpu command).

REVIEW:121944
CCBUG:341496