Move pen point difference calculation from shader code to JS

[adroitwhiz]

Resolves

Resolves #642

Proposed Changes

This changes the pen line code so that the difference between pen points is calculated in JS instead of in the shader code.

I also stopped passing u_penPoints into the fragment shader (it hasn't needed that since last time I fixed this; I just forgot to remove it then), and clarified some of the comments.

Reason for Changes

If the pen points' coordinates are both somewhat large, calculating the difference between them will be inaccurate if done using the lower floating-point precision on GPUs. This messes up vertex shader calculations further down the line.

I'm pleasantly surprised by how simple the fix was here--it shouldn't decrease performance at all.

Test Coverage

Tested manually on an iPad Mini 5 (A2133) that was previously exhibiting the issue.

[adroitwhiz]

After some investigation, it seems like the root cause of this is...

sigh...

Affected devices think that if you subtract a number from itself, you don't get 0.
This seems to be a shader miscompilation on Apple's side. This PR works around it. I'll try to contact Apple about this bug.

[fsih]

Nice! Looks reasonable to me!

[cwillisf]

LGTM!