Add STENCIL_BITS and DEPTH_BITS tests to webgl-depth-texture.html

Ensure DEPTH_BITS returns >= 16 bits for UNSIGNED_SHORT and >= 24 bits for UNSIGNED_INT
If there is stencil, ensure STENCIL_BITS reports >= 8 for UNSIGNED_INT_24_8_WEBGL.
Change DEPTH_BITS for UNSIGNED_INT from 24 to 16.

The depth texture extension says the following with regard to UNSIGNED_INT depth textures.
"As per the OpenGL ES spec, there is no guarantee that the OpenGL ES implementation will use the to determine how to store the depth texture internally. It may choose to downsample the 32-bit depth values to 16-bit or even 24-bit."

For the sake of not imposing new requirements, we'll change the minimum depth for UNSIGNED_INT from 24 to 16.
Add DEPTH_BITS and STENCIL_BITS requirements to WEBGL_depth_texture extension

UNSIGNED_SHORT --> Depth: >= 16 Stencil: 0
UNSIGNED_INT --> Depth: >= 16 Stencil: 0
UNSIGNED_INT_24_8_WEBGL --> Depth: >= 24 Stencil: >= 8
Address copying element array buffers in WebGL 2

Copying into element array buffers from buffers that contain other data
is disallowed in order to make sure index validation does not require
reading back buffer data from the GPU to the CPU. Copying element array
buffer data into other element array buffers can still be desirable, so
previous restrictions are relaxed so that element array buffers can be
bound to COPY_READ_BUFFER and COPY_WRITE_BUFFER.
Minor updates to WebGL 2 spec

Change the description to refer to GLES 3.0.3 spec instead of 3.0.2.

Add missing link target.

Add pixel buffer objects to WebGL 2.0 new feature list.
Make writing to gl_FragData[n != 0] fail compilation in WebGL 2

This should clear any ambiguity caused by some parts of the GLES 3.0
specification suggesting that writing to multiple outputs in GLSL ES 1.0
shaders would be possible, while section 4.2.1 says that calling
DrawBuffers only sets the output buffer for one output color for
GLSL ES 1.0 shaders.
Merge pull request #677 from NVIDIA/gl-fragdata-webgl2

Make writing to gl_FragData[n != 0] fail compilation in WebGL 2
Merge pull request #676 from NVIDIA/webgl2-minor-corrections

Minor updates to WebGL 2 spec
Merge pull request #670 from RafaelCintron/depthTexture

Add STENCIL_BITS and DEPTH_BITS tests to webgl-depth-texture.html
Merge pull request #675 from NVIDIA/webgl2-element-array-copy

Address copying element array buffers in WebGL 2
Merge branch 'master' of https://github.com/RafaelCintron/WebGL into depthTexture