Fix switching between const and non-const attributes

[rreusser]

This PR fixes switching between constant and non-constant attributes. See: #474. At the end of the day, the simple problem was that constant attributes set some properties on the BINDING via

          CUTE_COMPONENTS.map(function (c, i) { 
            return BINDING + '.' + c + '=' + CONST_COMPONENTS[i] + ';'
          }).join(''),

but do not actively unset buffer. As a result, the the next call to a buffer attribute does not notice that buffer attributes were previously disabled and now must be enabled. The solution was to simply set BINDING.buffer=null when using constant attributes.

The other half of the debugging was working around a strange instanced arrays interaction detailed in #544.

Sorry it took so long to get to to the bottom of this, but thanks @dy for providing such a debuggable test case!

[rreusser]

Yeah, travis doesn't pass anymore for what seems like changes to the v8 API. I'm inclined to merge this anyway since I've tried pretty hard to ensure it passes the tests and since I don't have the bandwidth to spend a day or two on headless-gl.

[milcktoast]

@rreusser I've had to switch to using node ^8.x with headless-gl.

[rreusser]

Maybe .travis.yml just needs an update?

regl/.travis.yml

Lines 14 to 15 in 12cb14b

	node_js:
	- '6'