Add support for WebGL2 ReadPixels functions

Adds support for the new ReadPixels functions introduced with WebGL2
and the relevant PixelStorei parameters.

Reference: https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.10

components/canvas/Cargo.toml

@@ -33,7 +33,7 @@ num-traits = "0.2"
 raqote = {git = "https://github.com/jrmuizel/raqote", optional = true}
 pixels = {path = "../pixels"}
 servo_config = {path = "../config"}
-sparkle = "0.1.8"
+sparkle = "0.1.12"
 webrender = {git = "https://github.com/servo/webrender"}
 webrender_api = {git = "https://github.com/servo/webrender", features = ["ipc"]}
 webrender_traits = {path = "../webrender_traits"}

components/canvas/webgl_thread.rs

@@ -1166,6 +1166,17 @@ impl WebGLImpl {
                 );
                 sender.send(&pixels).unwrap();
             },
+            WebGLCommand::ReadPixelsPP(rect, format, pixel_type, offset) => unsafe {
+                gl.read_pixels_into_pixel_pack_buffer(
+                    rect.origin.x,
+                    rect.origin.y,
+                    rect.size.width,
+                    rect.size.height,
+                    format,
+                    pixel_type,
+                    offset,
+                );
+            },
             WebGLCommand::RenderbufferStorage(target, format, width, height) => {
                 gl.renderbuffer_storage(target, format, width, height)
             },