Only do the alpha swizzle on alpha textures

Avoids problems if swizzles are used elsewhere (like in the newer X11
plugins)
Add glad opengl loader
Move from glLoadGen to GLAD
Rewrite pulseaudio input

Now more flexible and easily extendable
Add xcomposite capture plugin
Some XComposite capture fixes
Fix color swapping in xcompcap
Fix Pulseaudio latency calculation