TextureAtlas should re-use texture coordinates for existing subregions

[shunter]

TextureAtlas.addSubRegion allocates a new BoundingRectangle with texture coordinates every time it's called. If coordinates have already been computed for the given image ID and subRegion, the existing coordinates (at the existing index) should be re-used.

[pjcozzi]

CC #172

[markerikson]

Would love to see this worked on during the summer 2017 code sprint! (Also linking for reference: #2319 )

[alberto-acevedo]

• 1 to get this finally fixed.

[pjcozzi]

Thanks for the input! We'll see if someone is interested at the sprint.