Always use highp for color attributes

[anandthakker]

Use highp for all color attributes, because as of mapbox/mapbox-gl-native#8267 two color components may be packed into a single float, a process which assumes that at least 16 bits are available.

Refs mapbox/mapbox-gl-native#8385

Launch Checklist

• briefly describe the changes in this PR
• post benchmark scores
• manually test the debug page

benchmark	master 9941dde
map-load	140 ms
style-load	115 ms
buffer	1,061 ms
fps	60 fps
frame-duration	5.2 ms, 0% > 16ms
query-point	1.21 ms
query-box	65.76 ms
geojson-setdata-small	6 ms
geojson-setdata-large	199 ms