user/spice-gtk: switch coroutine impl from ucontext to gthread

Merged Max Rees requested to merge sroracle/packages:fix/spice-gtk-coro into master

Fixes a failure on ppc32:

 8/12 spice-gtk / test-coroutine              FAIL     0.02 s (killed by signal 6 SIGABRT)

 --- command ---
 /usr/src/packages/user/spice-gtk/src/spice-gtk-0.38/output/tests/test-coroutine
 --- stdout ---
 # random seed: R02S414e77a117fac2e9cd10eee9a9f61d84
 1..3
 # Start of coroutine tests
 Bail out! GSpice:ERROR:../tests/coroutine.c:37:test_coroutine_simple: assertion failed (GPOINTER_TO_INT(result) == 0x42): (0 == 66)
 --- stderr ---
 **
 GSpice:ERROR:../tests/coroutine.c:37:test_coroutine_simple: assertion failed (GPOINTER_TO_INT(result) == 0x42): (0 == 66)
 -------
Edited by Max Rees

Merge request reports