auto merge of #1008 : deokjinkim/servo/glyph_advance_cache, r=jdm

To reduce number of FT_Load_Glyph call, use glyph advance cache.
Add summit demos.

summit2.html is a two pane demo of rust logo and matrix math.

summit3.html is a three pane demo adding longcat.

summit-fail.html is thw two pane demo with failing page on the right.