Switch to `highp` precision for equirectangular fragment shader.
Switch to `highp` precision for equirectangular fragment shader.

This resolves pixelation issues on some mobile devices (fixes #725).
It also may be related to issues #301, #496, #683, and #708.

mpetroff/pannellum,mpetroff/pannellum,mpetroff/pannellum,mpetroff/pannellum