swr: [rasterizer core] make all api functions call GetContext
swr: [rasterizer core] make all api functions call GetContext

Small api cleanup.  Make all api functions call GetContext instead
of locally casting handle.  Makes debugging easier by providing a
single point to track context changes.

Signed-off-by: Tim Rowley <54947742fe7299a578dbb84f6c782c6c4c668d8f@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler