Post formats: add 32px retina icons for both color themes, tweak the css, "crush" the sprites. Props melchoyce, saracannon, aaroncampbell. Fixes #24290, fixes #24331.

git-svn-id: https://core.svn.wordpress.org/trunk@24297 1a063a9b-81f0-0310-95a4-ce76da25c4cd