Remove RGB32F support from WEBGL_color_buffer_float

Rendering to RGB32F is not natively supported in OpenGL ES or DirectX, so
the decision was made to remove support for this also from
WEBGL_color_buffer_float after discussion on the public mailing list.

Also mark it as deprecated on WebGL info page.
Merge pull request #793 from NVIDIA/webgl-color-buffer-float-rgb32f

Remove RGB32F support from WEBGL_color_buffer_float