Instrument FreeType allocations.

This will allow reporting of memory usage by FreeType instances, which
are measured in the MiBs for LayoutWorker threads.

This change also makes FreeType allocations happen with jemalloc instead
of the system malloc, which is a good thing.

Finally, the change documents some dubiousness involving
FontContextHandle.

components/gfx/lib.rs

@@ -16,6 +16,7 @@
 #[macro_use]
 extern crate log;
 
+extern crate alloc;
 extern crate azure;
 #[macro_use] extern crate bitflags;
 extern crate fnv;