Bug 1090518. Fix crash during webgl-depth-texture.html conformance test. r=jrmuizel

The crash happens because we get a GL error back from glTexImage2D when we try
to glTexImage2D a depth texture in EnsureNoUnitializedImageData. Uploading
glTexImage2D is not supported with the ANGLE_depth_texture extension. We need
to use the framebuffer path.

--HG--
extra : rebase_source : db249208c3f13c05d3f603f85b5614885171a9fb