rdar://problem/15949546 Most of the work here is just validation. I assume that the 1.0.3 conformance suite will have a test for this call.
Created attachment 242341 [details] patch
Some hand testing of this patch shows that the validation is working correctly. However, I am having some issues getting the actual call to work properly when I have more than 0 samples. I suspect my test case is incorrect since I don't specify anywhere to render a multisample texture. (texImage2DMultisample) I suspect that without this call the mismatch between a nonmultisampled texture and a render buffer with enough storage for a multisampled frame buffer is causing issues, but I could be wrong.
I have indeed confirmed that the OpenGL call to ::glRenderBufferStorageMultisample is getting called as intended however.
After emailing the Khronos group I got the following response from Kenneth Russel: I don't mean to discourage your work but this is no small task either from a specification or implementation standpoint. It would be necessary to spec out a WebGL 1 extension combining the semantics of both the ANGLE_framebuffer_blit and ANGLE_framebuffer_multisample extensions. Users' code also has to be modified to take advantage of multisampled renderbuffers. I think the effort would be better spent getting a WebGL 2 implementation ready, which will include this functionality in the core API. Indeed renderBufferStorageMultisample has not been specced out and there are no 1.0.x conformance tests testing it. TLDR: I don't think we should do this because it never made it into the WebGL spec in the first place.
Comment on attachment 242341 [details] patch Cleared review? from attachment 242341 [details] so that this bug does not appear in http://webkit.org/pending-review. If you would like this patch reviewed, please attach it to a new bug (or re-open this bug before marking it for review again).