webgl: mobile glyph rendering issues

[Spongman]

0.8.0
re: #3769

@stalgiag reported webgl font rendering issues on android mobile devices:

The text/chinese manual test is still rendering oddly on my mobile device (Android 8, Galaxy s7). I don't know if that has to be addressed in this PR as it is "more broken" in master currently. Just thought it worth noting. Here is a screenshot:

it looks like it might be due to integer precision issues on that device:

If I change the fragment shader to use high precision floats and also do the integer substitution with floats (ie #if 1 in line 4) then it displays correctly.

[Spongman]

@stalgiag
can you try this in font.frag?

precision mediump float;

#if 0
  ...
#else
  // use native integer math
  precision highp int;
  ...

[stalgiag]

@Spongman that also works!

[Spongman]

ok, so until we have some framework for parameterizing shader compilation, i'm just going to force highp int on all devices. i wish there was some way to say 'int16' or determinine precision at compile time (eg sizeof()), oh well...