Implement the basic WebGL2 buffer data operations

Adds support for `bufferData`, `bufferSubData`, `copyBufferSubData`
and `getBufferSubData`.

Reference: https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.3

components/canvas/webgl_thread.rs

@@ -55,6 +55,7 @@ use sparkle::gl::Gl;
 use std::borrow::Cow;
 use std::cell::{Cell, RefCell};
 use std::rc::Rc;
+use std::slice;
 use std::sync::{Arc, Mutex};
 use std::thread;
 use surfman;
@@ -1013,6 +1014,26 @@ impl WebGLImpl {
             WebGLCommand::BufferSubData(buffer_type, offset, ref receiver) => {
                 gl::buffer_sub_data(gl, buffer_type, offset, &receiver.recv().unwrap())
             },
+            WebGLCommand::CopyBufferSubData(src, dst, src_offset, dst_offset, size) => {
+                gl.copy_buffer_sub_data(
+                    src,
+                    dst,
+                    src_offset as isize,
+                    dst_offset as isize,
+                    size as isize,
+                );
+            },
+            WebGLCommand::GetBufferSubData(buffer_type, offset, length, ref sender) => {
+                let ptr = gl.map_buffer_range(
+                    buffer_type,
+                    offset as isize,
+                    length as isize,
+                    gl::MAP_READ_BIT,
+                );
+                let data: &[u8] = unsafe { slice::from_raw_parts(ptr as _, length) };
+                sender.send(data).unwrap();
+                gl.unmap_buffer(buffer_type);
+            },
             WebGLCommand::Clear(mask) => {
                 gl.clear(mask);
             },

components/canvas_traits/webgl.rs

@@ -248,6 +248,8 @@ pub enum WebGLCommand {
     BindAttribLocation(WebGLProgramId, u32, String),
     BufferData(u32, IpcBytesReceiver, u32),
     BufferSubData(u32, isize, IpcBytesReceiver),
+    GetBufferSubData(u32, usize, usize, IpcBytesSender),
+    CopyBufferSubData(u32, u32, i64, i64, i64),
     Clear(u32),
     ClearColor(f32, f32, f32, f32),
     ClearDepth(f32),