win32u: Clear DC opengl drawable when releasing cached dce.
We won't be able to do it later on otherwise as get_dc_ptr will fail with a disabled DC, and release_dce will then not clear the drawable.
This will delay the drawable release until the dce is later reused, for instance for a different window, which may cause a complicated deadlock.
Fixes a regression from 980a2e3c which causes timeout in the opengl32 tests.
Edited by Rémi Bernon