Deduplicate the bitflags dependency

[SimonSapin]

#1359 will likely land with this tidy error silenced:

./Cargo.lock:1: duplicate versions for package "bitflags"
	found dependency on version 0.7.0
	but highest version is 0.8.2
	try upgrading with ./mach cargo-update -p bitflags:0.7.0
	The following packages depend on version 0.7.0:
		clap
		cocoa
		png
		webrender
		xml-rs

[Eijebong]

• Bump xml-rs to 0.6 in gl_generator and bump version brendanzab/gl-rs#419
• Update bitflags 0.7 -> 0.9 image-rs/image-png#56
• Update bitflags 0.8.0 -> 0.9 clap-rs/clap#966 (needs a release)
• Update bitflags and bump version cocoa-rs#163
• Bump bitflags to 0.9 and bump version core-graphics-rs#86