LUMINANCE and ALPHA textures should not be renderable

Check that attaching these texture types results in an incomplete
framebuffer instead of trying to render to that framebuffer. This helps
to ensure that unspecified rendering to these texture types is not
supported in any implementation, maintaining compatibility.

For reference, see OpenGL ES 2.0 spec section 4.4.5 Framebuffer
Completeness, specifically table 4.5. "Renderbuffer image formats", which
enumerates all renderable formats. Rendering to other formats is not
specified.