Unbind Attributes after use

[hugoam]

This pursue (and finishes, I presume) the work from previous pull request #1512 to make bgfx play nice with WebGL 2.0
In some cases, WebGL will produce errors because some attributes are still bound, but they are not bound to any buffers anymore.

This is due to bgfx not unbinding them after finishing with a program. This patch fixes it by always unbinding attributes when binding a new program, and the last bound program at the end of the draw loop.

I hesitated to add this behavior under #IF BX_PLATFORM_EMSCRIPTEN guards, but I assumed the performance would be more than negligible so it would not hurt to add it to the general path.

(Some reference of browser code enforcing this:
https://dxr.mozilla.org/mozilla-central/rev/a461fe03fdb07218b7f50e92c59dde64b8f8a5b0/dom/canvas/WebGLProgram.cpp#480)

[bkaradzic]

Where do you update m_unboundUsedAttrib?

[hugoam]

We use the same m_unboundUsedAttrib that were setup in bindAttributes. It's the same logic used by bindAttributesEnd except in reverse, if it equals Attrib::Count then we know they were bound so we need to unbind/disable them.

m_unboundUsedAttrib is erased in bindAttributesBegin when the same program is used again, so it's safe to use it when unbinding it since that happens before. Unless I'm mistaken, bindAttributesBegin cannot logically be called in between.