renderbufferStorage with RGB565 format causes panic in GL thread

[jdm]

<canvas id="canvas"></canvas>
<script>
  var gl = document.getElementById('canvas').getContext('webgl');
  var buffer = gl.createRenderbuffer();
  gl.bindRenderbuffer(gl.RENDERBUFFER, buffer);
  gl.renderbufferStorage(gl.RENDERBUFFER, gl.RGB565, 1, 1);
  console.log(gl.getError());
</script>

This yields:

ERROR 2018-07-26T16:43:52Z: canvas::webgl_thread: Last GL operation failed: RenderbufferStorage(36161, 36194, 1, 1)
assertion failed: `(left == right)`
  left: `1280`,
 right: `0`: Unexpected WebGL error: 0x500 (1280) (thread WebGLThread, at components/canvas/webgl_thread.rs:1131)

The RGB565 format is allowed by the spec and checked in the conformance tests. This does not affect headless builds using OSMesa, so I'm going to see if Firefox's implementation does anything special here.

[jdm]

Aha: https://searchfox.org/mozilla-central/rev/943a6cf31a96eb439db8f241ab4df25c14003bb8/dom/canvas/WebGLRenderbuffer.cpp#93-106

[jdm]

I have a local branch that addresses this.