Fix the NULL deref that occured when blitting to offscreen targets and
select the proper gl drawing buffer instead.

This change was triggered because of a bug in libsane-hpaio.so, namely
that it relies on libsane.so being dlopen'd RTLD_GLOBAL so that
sanei_init_debug can be resolved.

user32: Add several more MsgWaitForMultipleObjects tests and split them out from the main message tests.

include: Add a definition for REGCLS_SURROGATE and add declarations for CoRegisterSurrogate and CoGetObjectContext.

Also make the conversions more strict against invalid input data.

Also some spelling fixes.