Bug 1349067 - Make gl as current at WebGL2 ReadBuffer(); r=ethlin,jgilbert

MozReview-Commit-ID: 5yrsuhilb1N

--HG--
extra : rebase_source : e22d5bf358ac1e73f4073d39f99ae795d6ce65a2

dom/canvas/WebGL2ContextFramebuffers.cpp

@@ -316,6 +316,8 @@ WebGL2Context::ReadBuffer(GLenum mode)
     if (IsContextLost())
         return;
 
+    gl->MakeCurrent();
+
     if (mBoundReadFramebuffer) {
         mBoundReadFramebuffer->ReadBuffer(funcName, mode);
         return;