Revert "Revert "Merge pull request #2609 from brson/parallel-render""

This reverts commit 3f8f065b76d034803bfb666899ef07afce0a1cd9.
gfx: Don't crash if there is no graphics metadata.
auto merge of #2687 : pcwalton/servo/parallel-render, r=larsbergstrom

Fixes the Mac crash.

r? @larsbergstrom