cross-fade between texture zooms to maintain texture density

[meetar]

Problem

When using a texture in a material, the default texture scale is [1, 1], which means a texture isn't visible at native pixel density until z21 (on my retina screen):

http://tangrams.github.io/texture-demos/problem-case/ ☝️ Zoom in until you see the textures scale properly, then adjust the slider and check out other zoom levels.

Proposal

Here's a method of cross-fading between two copies of the texture for each zoom:

http://tangrams.github.io/texture-demos/?url=scenes/texture-tiles.yaml#12/37.8090/-122.2220

This could be a more reasonable default scaling option for textures, maybe with a syntax addition of scale: crossfade.

[bcamper]

Seems like the proposal demo isn't currently loading.

[meetar]

Yep – fixed with https://github.com/tangrams/texture-demos/commit/a541532a2ed5a9193d38409ef21a7927b50728ee