Need highp precision for gamma values on mobile devices. lowp triggers aliasing artifacts at larger font sizes.
Note that this change is a hand-edit to the generated shader sources, rather than pulling https://github.com/mapbox/mapbox-gl-js/pull/4275 and running the generator script. The reason for that is that the upstream shader now assumes support for DDS properties that gl-native does not yet support. Once that support lands in gl-native, we can regenerate the shader source.
8f1c580ea1