Show valid framebuffer attachment combinations

[pjcozzi]

GL does not guarantee that all combinations of framebuffer attachments are supported. For example, without ANGLE on Windows XP, a stencil or depth/stencil renderbuffer cannot be attached to a framebuffer.

We should select a handful of common attachment combinations and report if they are supported, e.g.,

• Texture RGBA, Renderbuffer DEPTH_COMPONENT16
• Texture RGBA, Renderbuffer STENCIL_INDEX8
• Texture RGB, Renderbuffer DEPTH_COMPONENT16
• Texture RGB, Renderbuffer STENCIL_INDEX8

For more information, search for "CheckFramebufferStatus" or "FRAMEBUFFER_UNSUPPORTED" in the GL ES spec: www.khronos.org/registry/gles/specs/2.0/es_full_spec_2.0.25.pdf.

Section 6.5 in the WebGL spec may also be interesting: www.khronos.org/registry/webgl/specs/latest/