Make Pholio selection reticle prettier

Summary:
The selection reticle in Pholio is functional but not very pretty right now. Make it look a little nicer.

• Using box-sizing: border-box; allows us to get rid of the x - 1 and y - 2 stuff.
• I draw the reticle with two elements: one mostly-transparent which creates a fill, and one fully opaque to create a strong dashed border.

Test Plan: {F31803}

Reviewers: ljalonen, chad

Reviewed By: chad

CC: aran, kchr

Differential Revision: https://secure.phabricator.com/D4836