Re-work the makeSomeOpenGLContextCurrent logic into a shared function
with the existing AcquireContext logic in the SalOpenGLGraphics impl.
Use an OpenGLVCLContextZone placeholder to do both context and zone
management - to include destructors into the zone.

Fix a number of error cases around SalBitmaps where we did not have
a GL context associated when allocating, and/or freeing textures..

Don't drag a (potentially) slower context around in the
OpenGLSalBitmap when we're going to check / fetch a better VCL
context anyway.

Change-Id: Ibbb2358c47156cd078ad28b6aad4f03af36aaf23
Reviewed-on: https://gerrit.libreoffice.org/21127Reviewed-by: Michael Meeks <michael.meeks@collabora.com>
Tested-by: Michael Meeks <michael.meeks@collabora.com>