The surface set during bindTexImage should be unset during
releaseTexImage. Failing to do so may keep a dangling pointer
to the surface, which can cause a crash later on (this was hit
during the testing of new unit tests which  will be added soon).

Change-Id: Ic258fff8c880d449add825c5e9185435cc4ea419
Reviewed-on: https://swiftshader-review.googlesource.com/17548Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>