Add support for some more WebGL2 renderbuffer functions

Adds support for the following WebGL2 calls:

- `RenderbufferStorageMultisample`
- `GetInternalFormativ`

See: https://www.khronos.org/registry/webgl/specs/latest/2.0/#3.7.5

components/canvas_traits/webgl.rs

@@ -330,6 +330,7 @@ pub enum WebGLCommand {
     TransformFeedbackVaryings(WebGLProgramId, Vec<String>, u32),
     PolygonOffset(f32, f32),
     RenderbufferStorage(u32, u32, i32, i32),
+    RenderbufferStorageMultisample(u32, i32, u32, i32, i32),
     ReadPixels(Rect<u32>, u32, u32, IpcBytesSender),
     ReadPixelsPP(Rect<i32>, u32, u32, usize),
     SampleCoverage(f32, bool),
@@ -458,6 +459,7 @@ pub enum WebGLCommand {
     GetCurrentVertexAttrib(u32, WebGLSender<[f32; 4]>),
     GetTexParameterFloat(u32, TexParameterFloat, WebGLSender<f32>),
     GetTexParameterInt(u32, TexParameterInt, WebGLSender<i32>),
+    GetInternalFormatIntVec(u32, u32, InternalFormatIntVec, WebGLSender<Vec<i32>>),
     TexParameteri(u32, u32, i32),
     TexParameterf(u32, u32, f32),
     DrawArrays {
@@ -913,6 +915,14 @@ parameters! {
     }
 }
 
+parameters! {
+    InternalFormatParameter {
+        IntVec(InternalFormatIntVec {
+            Samples = gl::SAMPLES,
+        }),
+    }
+}
+
 #[macro_export]
 macro_rules! gl_enums {
     ($(pub enum $name:ident { $($variant:ident = $mod:ident::$constant:ident,)+ })*) => {