Auto merge of #25975 - mmatyas:webgl_fns_read_draw_buffers_p2, r=jdm

Do not try to read pixels from an FBO without read buffer

A follow up to #25905, this adds another check to the WebGL2 ReadPixels implementation to fix
an OpenGL invalid operation crash when the method is called on a bound framebuffer that has no read buffer.

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cc @jdm @zakorgy

However, it seems there's an issue with the headless mode: when ReadBuffer is called on the default framebuffer with the value `GL_BACK`, like [here](https://github.com/servo/servo/blob/e1103176e3de8a8c0996d1d60c092cfd8f60e805/tests/wpt/webgl/tests/conformance2/renderbuffers/readbuffer.html#L77), in headless mode an invalid operation is generated. In non-headless mode the whole test completes successfully:
![sshot](https://user-images.githubusercontent.com/4354863/76845770-b0ffbd80-683f-11ea-8103-a81512188808.png)

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

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1 files changed, 5 insertions, 1 deletions

diff --git a/components/script/dom/webgl2renderingcontext.rs b/components/script/dom/webgl2renderingcontext.rs
index c0c3aab5ad6..7b1a68b475a 100644
--- a/components/script/dom/webgl2renderingcontext.rs
+++ b/components/script/dom/webgl2renderingcontext.rs
@@ -349,7 +349,11 @@ impl WebGL2RenderingContext {
         }
 
         let fb_slot = self.base.get_draw_framebuffer_slot();
-        if fb_slot.get().is_none() && self.default_fb_readbuffer.get() == constants::NONE {
+        let fb_readbuffer_valid = match fb_slot.get() {
+            Some(fb) => fb.attachment(fb.read_buffer()).is_some(),
+            None => self.default_fb_readbuffer.get() != constants::NONE,
+        };
+        if !fb_readbuffer_valid {
             return self.base.webgl_error(InvalidOperation);
         }