Fix multiview for WebXRManager, restoring and updating lost changes

[mrxz]

Based on #24. Turns out there were a couple more issues that needed to be resolved to get multiview working again.

• The textures.runDeferredUploads() call got lost
• The logic in setRenderTargetTextures changed in Three.js upstream, making the multiview specific changes no longer needed
• The cache key in WebGLPrograms used the same index for numMultiviewViews as alphaToCoverage. Most likely one got added upstream and the numMultiviewViews wasn't incremented.

[KooIaIa]

I built this version of three, added it to a-frame, built aframe, and ran this example https://aframe.io/examples/showcase/handtracking/

Set multiviewStereo: true. Ran great! No visual bugs or performance issue.

[KooIaIa]

Actually I do get 1 warning when using this though it seems to work ok:

WebGL: INVALID_ENUM: texParameter: invalid parameter
a-scene.js:289

[dmarcos]

Thank you!