gpu.c: create_tile: improve error handling
gpu.c: create_tile: improve error handling

In particular, avoid the use of assert, returning NULL on failure instead.
This allows us to avoid including assert.h when we extract out these
functions to a separate file.

Signed-off-by: Sven Verdoolaege <e5350bbed4977f5eb8ae1dc6abd9ae59d21ace75@kotnet.org>

nicolasvasilache/ppcg,nicolasvasilache/ppcg,Meinersbur/ppcg,Meinersbur/ppcg