provide better time precision to shaders

[Aeva]

Currently m.grl provides the float mgrl_frame_start uniform, who's value is performance.now()/1000.0. Supposedly performance.now() provides accuracy up to 5 microseconds (expressed as milliseconds).

Converting seconds and then dumping it into a float is a fast way to loose precision, and makes mgrl_frame_start not as useful for animation. The reason for this, is that as a float's magnitude increases, the fractional part of the float becomes choppy.

It is tempting instead to provide instead just the whole number of milliseconds as an integer instead, and leave it as an exercise for the shaders to extract the relevant fractional component out of that.

It might also be useful to provide the number of seconds as an integer, and the fractional component as a separate floating point.

So, for this ticket:

• [ ] replace float mgrl_frame_start with something better
• [ ] update all demos to use the new thing

Some things to consider:

• according to the internet, glsl floats can be either 8, 10, or 16 bits, depending on which precision qualifier.
• according to my gl.getShaderPrecisionFormat(gl.FRAGMENT_SHADER, gl.HIGH_FLOAT) on my intel laptop, precision: 23
• it is probably safe to assume that on all desktops m.grl is likely to run on, highp is available
• so, assuming that is typical, integers start to lose precision at 2**24, giving me about 194 days of runtime before it should be noticable in animations.
• just switching to milliseconds and no fractional var would give about 4 hours
• it looks like results can vary wildly though :| especially on mobile chipsets

A possible approach:

1. send int seconds + float fraction of seconds to shader
2. produce texture for seconds and seconds+1
3. lerp results with fraction of seconds as alpha value

[Aeva]

With some testing, it looks like the glsl sin function drops a lot of precision. Without doing anything fancy, it looks like a highp float is fine for a day of precision for time if I do something like this first:

  float speed = 1.3;
  float two_pies = 6.283185307179586;
  float time = fract(mgrl_frame_start*speed/two_pies)*two_pies;
  float wave = sin((dist - time) * frequency);

So, nothing really to do here.