gl: numerous memory fixes (use-after-free, leaks, missing NULL-ify)

commit a171eb80

glcolorconvert: error out if memory context is different than ours

commit 2a4a3aa0

gltestsrc: NULL out the functions used

Fixes segfault when the display changes

commit 1abd05db

glviewconvert: fix reset context for GL context change

We need to remove GL resources from the old context instead of the new
GL context.  The two GL context may not even be shared.

commit af198635

glviewconvert: fix a use-after-free with allocation params

_free(params) followed by a params->align is a bad idea :)

commit cbffe13d

glviewconvert: also remove GL buffers on a reset

Fixes a possible memory leak on renegotiation

commit 9461d178

gl/basesrc: call gl_stop when shutting down

Fixes multiple leaks