Optimize image assets

[frederoni]

Noticed that there are room for improvements in terms of image assets:

• Compass.png is incorrect scale (2x = 1,8x, 3x = 2,81) (3x downscaling should be handled automatically) which results in half pixels when loading from resources.
• Logo is positioned on half pixels which prevents pixel perfect rendering.
• App Thinning cannot slice resources unless they're in an asset catalog. (would save ~12KB on 2x devices) (Cocoapods supports asset catalogs as of version 1.0)

[friedbunny]

Sounds great. Asset catalogs are iOS 7+, so we’re good there.

[1ec5]

Logo is positioned on half pixels which prevents pixel perfect rendering.

We mitigate this with fractional alignment rect insets. It would be great to get these images and their insets into an asset catalog.

[1ec5]

Using an asset catalog would also allow us to replace the existing default_marker.png with default_marker.pdf, which is scalable.

[1ec5]

#8703 proposes to replace these PNGs with PDFs.

[friedbunny]

Fixed by #9060, will be in v3.6.0-beta.1.