Clarify constants and calculations in SDF rendering

[anandthakker]

There are several constants and calculations in draw_symbol.js and the symbol_sdf shaders that are difficult to interpret / understand. I'm pretty much with @lucaswoj when he says:

I prefer a very clear constant name (ideally googleable) to a magic number, and a magic number to a cryptic constant name.

(though I'd say a semi-clear constant name with a one-line comment to explain it still beats a magic number in my book)

[anandthakker]

cc @lbud @ansis based on my best guess from reading through some of the git history in these files.

Examples of inscrutable constant names:

• SDF_PX (was sdfPx)
• gammaScale => u_gamma_scale (compare w/ v_gamma_scale :scream`)
• EDGE_GAMMA (was just u_gamma I think)
• blurOffset (now inlined as 6.0 in the frag shader)
• buff (was u_buffer) (btw, is it a coincidence that 6/8 (present in calculation for the halo version of this value) = 0.75 = (265 - 64)/256 (the non-halo version of this value)?)

[anandthakker]

Thanks to a tip from @ChrisLoer, noticed that TinySDF appears to have some default constant values that line up with these:

    this.fontSize = fontSize || 24;
    this.buffer = buffer === undefined ? 3 : buffer;
    this.cutoff = cutoff || 0.25;
    this.fontFamily = fontFamily || 'sans-serif';
    this.radius = radius || 8;

https://github.com/mapbox/tiny-sdf/blob/master/index.js#L8-L12