Auto merge of #57 - emilio:canvas-leaks, r=glennw
Auto merge of #57 - emilio:canvas-leaks, r=glennw

api: Add an api method to deallocate images.

Allows taking rid of some always-growing memory consumption, specially noticeable in 2d canvas.

r? @glennw

servo/webrender,servo/webrender_traits,servo/webrender,servo/webrender,alanamramjit/webrender,servo/webrender,servo/webrender,servo/webrender,alanamramjit/webrender