XCB/window: map Super and AltGr keys to Meta and Alt modifiers resp.
XCB/globalhostkeys: add support for meta modifier
keys: define Print and Pause keys
Qt: map Print and Pause keys
XCB: map Print and Pause keys
gl: remove hard-coded platform to window type mappings

This should fix compilation on non-Apple Darwin and plausibly addresses
hypothetical future support for Mesa's shared GLAPI.
contrib: don't use the gas-preprocessor when compiling for the iOS simulator
Contribs: update matroska to 1.4.1
Contribs: update GnuTLS to 3.1.16