If the user set a wrapS or wrapT parameter we should warn him if his
texture is not power of two.  We should also warn him if his minFilter
is changing.

Also trying to only call gl.getExtension() when needed.

WebGLRenderer: Extensions code clean up. Need to refactor this so we get the extension only when needed...

WebGLRenderer: Set _currentGeometryGroupHash to -1 after deallocating geometry just in case (temporal hack). Fixes #5421.

  - convert available format to real Array
  - test if texture glFormat is found in supported formats

note : the test against supported format before upload log an explicit warning but can't prevent gl warnings during draw call (texture is invalid)

  - Split DDSParser, put common parsing process in abstract CompressedTextureLoader
  - added PVRLoader based on CompressedTextureLoader
  - added feature detection and PVRTC related gl contants in Three / WebglRenderer
  - added a sample page

Not pretty but better than nothing I guess.

WebGLRenderer: Deallocate Object when removed from scene graph (no need for object.dispose()). See #5269.

Reverted 4439d901. Implemented material.visible in Projector. See #5256.