Bug 1237637 - Disallow negative levels for framebuffer attachments. - r=jrmuizel

dom/canvas/WebGLContextGL.cpp

@@ -537,6 +537,11 @@ WebGLContext::FramebufferTexture2D(GLenum target,
     if (!ValidateFramebufferTarget(target, "framebufferTexture2D"))
         return;
 
+    if (level < 0) {
+        ErrorInvalidValue("framebufferTexture2D: level must not be negative.");
+        return;
+    }
+
     if (!IsWebGL2() && level != 0) {
         ErrorInvalidValue("framebufferTexture2D: level must be 0.");
         return;