Add initial support for WebGL2 read framebuffer

Adds support for binding to the read framebuffer slot and querying
its status.

components/script/dom/webgl2renderingcontext.rs

@@ -503,6 +503,13 @@ impl WebGL2RenderingContextMethods for WebGL2RenderingContext {
                     self.current_transform_feedback.get()
                 );
             },
+            // NOTE: DRAW_FRAMEBUFFER_BINDING is the same as FRAMEBUFFER_BINDING, handled on the WebGL1 side
+            constants::READ_FRAMEBUFFER_BINDING => unsafe {
+                return optional_root_object_to_js_or_null!(
+                    *cx,
+                    &self.base.get_read_framebuffer_slot().get()
+                );
+            },
             _ => {},
         }
 
@@ -650,7 +657,32 @@ impl WebGL2RenderingContextMethods for WebGL2RenderingContext {
 
     /// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
     fn BindFramebuffer(&self, target: u32, framebuffer: Option<&WebGLFramebuffer>) {
-        self.base.BindFramebuffer(target, framebuffer)
+        handle_potential_webgl_error!(
+            self.base,
+            self.base.validate_new_framebuffer_binding(framebuffer),
+            return
+        );
+
+        let (bind_read, bind_draw) = match target {
+            constants::FRAMEBUFFER => (true, true),
+            constants::READ_FRAMEBUFFER => (true, false),
+            constants::DRAW_FRAMEBUFFER => (false, true),
+            _ => return self.base.webgl_error(InvalidEnum),
+        };
+        if bind_read {
+            self.base.bind_framebuffer_to(
+                target,
+                framebuffer,
+                &self.base.get_read_framebuffer_slot(),
+            );
+        }
+        if bind_draw {
+            self.base.bind_framebuffer_to(
+                target,
+                framebuffer,
+                &self.base.get_draw_framebuffer_slot(),
+            );
+        }
     }
 
     /// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.7
@@ -2221,7 +2253,20 @@ impl WebGL2RenderingContextMethods for WebGL2RenderingContext {
 
     /// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.6
     fn CheckFramebufferStatus(&self, target: u32) -> u32 {
-        self.base.CheckFramebufferStatus(target)
+        let fb_slot = match target {
+            constants::FRAMEBUFFER | constants::DRAW_FRAMEBUFFER => {
+                self.base.get_draw_framebuffer_slot()
+            },
+            constants::READ_FRAMEBUFFER => &self.base.get_read_framebuffer_slot(),
+            _ => {
+                self.base.webgl_error(InvalidEnum);
+                return 0;
+            },
+        };
+        match fb_slot.get() {
+            Some(fb) => fb.check_status(),
+            None => constants::FRAMEBUFFER_COMPLETE,
+        }
     }
 
     /// https://www.khronos.org/registry/webgl/specs/latest/1.0/#5.14.7

components/script/dom/webglrenderbuffer.rs

@@ -96,20 +96,24 @@ impl WebGLRenderbuffer {
         if !self.is_deleted.get() {
             self.is_deleted.set(true);
 
+            let context = self.upcast::<WebGLObject>().context();
+
             /*
-            If a renderbuffer object is deleted while its image is attached to the currently
-            bound framebuffer, then it is as if FramebufferRenderbuffer had been called, with
-            a renderbuffer of 0, for each attachment point to which this image was attached
-            in the currently bound framebuffer.
-            - GLES 2.0, 4.4.3, "Attaching Renderbuffer Images to a Framebuffer"
-             */
-            let currently_bound_framebuffer =
-                self.upcast::<WebGLObject>().context().bound_framebuffer();
-            if let Some(fb) = currently_bound_framebuffer {
+            If a renderbuffer object is deleted while its image is attached to one or more
+            attachment points in a currently bound framebuffer object, then it is as if
+            FramebufferRenderbuffer had been called, with a renderbuffer of zero, for each
+            attachment point to which this image was attached in that framebuffer object.
+            In other words,the renderbuffer image is first detached from all attachment points
+            in that frame-buffer object.
+            - GLES 3.0, 4.4.2.3, "Attaching Renderbuffer Images to a Framebuffer"
+            */
+            if let Some(fb) = context.get_draw_framebuffer_slot().get() {
+                let _ = fb.detach_renderbuffer(self);
+            }
+            if let Some(fb) = context.get_read_framebuffer_slot().get() {
                 let _ = fb.detach_renderbuffer(self);
             }
 
-            let context = self.upcast::<WebGLObject>().context();
             let cmd = WebGLCommand::DeleteRenderbuffer(self.id);
             if fallible {
                 context.send_command_ignored(cmd);