3d rendered bezier curve args get overwritten, fixes #3344

[unjust]

Reorder argument setting to bezier in 3d mode, so original arg values aren't overwritten.

In the code below:
https://github.com/processing/p5.js/blob/master/src/webgl/3d_primitives.js#L1088
x2 should = z1, but assigning y2 = x2 afterwards is basically assigning y2 = z1.

Demoing the bug:

Having a bezier with all z coords set to 0 should result in the same curve as a bezier with no z;

Like this (with changes from this branch):

[unjust]

fixes issue #3344

[unjust]

@lmccart this should probably be fixed in the same way (using arguments) that was recommended earlier yes?

or is there a better way that will fit into the es6 transition?

[unjust]

@lmccart ping! what are the next steps on this?

[stalgiag]

Sorry for letting this sit for so long. Thanks @unjust!