Propagate debug flag in Makefiles.
Adding renderer drawing category to profiler.
Warning police.
Refactor renderer into single function.

This removes the task per tile rendering and instead renders tiles
serially. This also unwraps the rendering into a single function so that it's
much clearer.
auto merge of #523 : metajack/servo/gpu-profiling, r=pcwalton

This makes the rendering code serial instead of parallel since it now just dispatches commands to the GPU.

It also adds some more profiling data.

r? @pcwalton