Add retina support to raster tiles (@2x) + texture compression in Unity

[david-rhodes]

Our API supports requesting textures at 2x, but our SDK does not help with formatting the URL to do so.

We should also offer the ability to compress at runtime in Unity to save memory (at the cost of CPU time during compression), and investigate the ability to load raw data in a compressed format.

[david-rhodes]

Done: #48, mapbox/mapbox-sdk-cs#77

[david-rhodes]

Original

Retina

[david-rhodes]

Reopening because I just found out that the final texture format changes automatically after texture.LoadImage();. In order to actually support different types, we may need to investigate LoadRawTextureData.

Worst case scenario, I'll remove texture format and just have a boolean for compress textures.