Add support for fixed rate compression extensions

Signed-off-by: Louis-Francis Ratté-Boulianne <lfrb@collabora.com>

wrappers/glcaps: Check config availablility before dereferencing

Fixes: #786

appveyor: Update GitHub deployment token.

libbacktrace: Explictly set HAVE_STDINT_H.

Fixes #727

appveyor: Set shorter job names.

dlltrace: Implement dll path override.

Implement env var APITRACE_FORCE_MODULE_PATH which allows the user to
force override the directory to the dirs we want to trace against.This
is useful for tracing against reimplementations (ie. d9vk/dxvk) on their
native platforms.

appveyor: Update github release token.

libpng: Update to 1.6.19.

gui: Fix erasing elements from list during iteration.

Fixes issue #357.

gltrace: Fix PBO unpacking on core contexts.

gltrace::PROFILE_COMPAT was a misnomer -- it actually meant desktop
profile.  So b0c5972 broke PBO for core
contexts.