Add glow::Painter::intermediate_fbo()

This allow callbacks to restore to the correct framebuffer
after using their own temporary FBO.

See discussion in https://github.com/emilk/egui/issues/1744

egui_glow/CHANGELOG.md

@@ -7,6 +7,7 @@ All notable changes to the `egui_glow` integration will be noted in this file.
 * `EguiGlow::new` now takes an `EventLoopWindowTarget<E>` instead of a `winit::Window` ([#1634](https://github.com/emilk/egui/pull/1634)).
 * Use `Arc` for `glow::Context` instead of `Rc` ([#1640](https://github.com/emilk/egui/pull/1640)).
 * Fixed `glClear` on WebGL1 ([#1658](https://github.com/emilk/egui/pull/1658)).
+* Add `Painter::intermediate_fbo` which tells callbacks where to render. This is only needed if the callbacks use their own FBO:s and need to know what to restore to.
 
 
 ## 0.18.1 - 2022-05-05

egui_glow/src/painter.rs

@@ -248,6 +248,19 @@ impl Painter {
         self.max_texture_side
     }
 
+    /// The framebuffer we use as an intermediate render target,
+    /// or `None` if we are painting to the screen framebuffer directly.
+    ///
+    /// This is the framebuffer that is bound when [`egui::Shape::Callback`] is called,
+    /// and is where any callbacks should ultimately render onto.
+    ///
+    /// So if in a [`egui::Shape::Callback`] you need to use an offscreen FBO, you should
+    /// then restore to this afterwards with
+    /// `gl.bind_framebuffer(glow::FRAMEBUFFER, painter.intermediate_fbo());`
+    pub fn intermediate_fbo(&self) -> Option<glow::Framebuffer> {
+        self.post_process.as_ref().map(|pp| pp.fbo())
+    }
+
     unsafe fn prepare_painting(
         &mut self,
         [width_in_pixels, height_in_pixels]: [u32; 2],

egui_glow/src/post_process.rs

@@ -176,6 +176,11 @@ impl PostProcess {
         })
     }
 
+    /// What we render to.
+    pub(crate) fn fbo(&self) -> glow::Framebuffer {
+        self.fbo
+    }
+
     pub(crate) unsafe fn begin(&mut self, width: i32, height: i32) {
         if (width, height) != self.texture_size {
             self.gl