 FrameUniforms.resolution gets consistent precision between VS/FS (#122)
 FrameUniforms.resolution gets consistent precision between VS/FS (#122)

* Add option USE_EXTERNAL_GLES3

Causes gl_headers to use GLES, even on desktop.
Causes all ContextManagers to be elided;
users are expected to provide their own.

* FrameUniforms.resolution gets consistent precision between VS/FS

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