sokol_gfx.h: add support for half-float vertex attributes

[darkuranium]

I had a need for half-float vertex attributes to save on memory, and figured this would be a useful addition upstream. Feel free to ignore this pull request if you disagree.

From my own research, it should be available on nearly all GLES2 devices (94.5% coverage according to http://opengles.gpuinfo.org/listextensions.php), and all WebGL2/GLES3 devices.

Note: A useful addition would be a uint16_t sg_half_from_float(float) / float sg_float_from_half(uint16_t) utility function pair, to help with float16⇔float32 conversion.
It is not part of this pull request (mainly since it's not strictly speaking part of sokol_gfx's core purpose), but I do have code available, if needed.

The SG_VERTEXFORMAT_HALF{2,4} enums are positioned at the end of the existing enum list, for integer-backwards-compatibility reasons. Feel free to reshuffle these if desired.

[floooh]

Checking this out now. I think the helper functions to convert between float16 and float32 are out of the scope of sokol-gfx though. If there ever would be a sokol_math.h header, it would better fit in there ;)

[floooh]

...and merged. Many thanks!