Move set_index_buffer FFI functions back into wgpu.

It's very odd to have almost all the render pass and compute pass ffi
functions in `wgpu` except for the `set_index_buffer` functions, which
live in Firefox. I'd like to remove these from Firefox and put them
back next to their companions.

These functions were originally removed from wgpu in gfx-rs#1077, because
wgpu-native has its own incompatible version of IndexFormat (see that
PR for details). However, with wgpu-native#85, that code was removed,
so having these functions in `wgpu` should be no longer be a problem
for wgpu-native.

wgpu-core/src/command/bundle.rs

@@ -1219,7 +1219,7 @@ pub mod bundle_ffi {
     use super::{RenderBundleEncoder, RenderCommand};
     use crate::{id, RawString};
     use std::{convert::TryInto, slice};
-    use wgt::{BufferAddress, BufferSize, DynamicOffset};
+    use wgt::{BufferAddress, BufferSize, DynamicOffset, IndexFormat};
 
     /// # Safety
     ///
@@ -1283,6 +1283,17 @@ pub mod bundle_ffi {
         });
     }
 
+    #[no_mangle]
+    pub extern "C" fn wgpu_render_bundle_set_index_buffer(
+        encoder: &mut RenderBundleEncoder,
+        buffer: id::BufferId,
+        index_format: IndexFormat,
+        offset: BufferAddress,
+        size: Option<BufferSize>,
+    ) {
+        encoder.set_index_buffer(buffer, index_format, offset, size);
+    }
+
     /// # Safety
     ///
     /// This function is unsafe as there is no guarantee that the given pointer is

wgpu-core/src/command/render.rs

@@ -2005,7 +2005,7 @@ pub mod render_ffi {
     };
     use crate::{id, RawString};
     use std::{convert::TryInto, ffi, num::NonZeroU32, slice};
-    use wgt::{BufferAddress, BufferSize, Color, DynamicOffset};
+    use wgt::{BufferAddress, BufferSize, Color, DynamicOffset, IndexFormat};
 
     /// # Safety
     ///
@@ -2068,6 +2068,17 @@ pub mod render_ffi {
         });
     }
 
+    #[no_mangle]
+    pub extern "C" fn wgpu_render_pass_set_index_buffer(
+        pass: &mut RenderPass,
+        buffer: id::BufferId,
+        index_format: IndexFormat,
+        offset: BufferAddress,
+        size: Option<BufferSize>,
+    ) {
+        pass.set_index_buffer(buffer, index_format, offset, size);
+    }
+
     #[no_mangle]
     pub extern "C" fn wgpu_render_pass_set_blend_constant(pass: &mut RenderPass, color: &Color) {
         pass.base