Get capture mostly working

`File::create` isn't available on wasm, so we can't actually write the
output image anywhere unless we target something else as the file
system (e.g. local storage)

examples/capture/main.rs

@@ -96,7 +96,12 @@ async fn run() {
     // be called in an event loop or on another thread.
     device.poll(wgpu::Maintain::Wait);
 
-    // Write the buffer as a PNG
+    // If a file system is available, write the buffer as a PNG
+    let has_file_system_available = cfg!(not(target_arch = "wasm32"));
+    if !has_file_system_available {
+        return;
+    }
+
     if let Ok(mapping) = buffer_future.await {
         let mut png_encoder = png::Encoder::new(File::create("red.png").unwrap(), size, size);
         png_encoder.set_depth(png::BitDepth::Eight);
@@ -109,8 +114,19 @@ async fn run() {
     }
 }
 
+#[cfg(target_arch = "wasm32")]
+#[cfg_attr(target_arch = "wasm32", wasm_bindgen::prelude::wasm_bindgen(start))]
+pub fn wasm_main() {
+    console_log::init().expect("could not initialize log");
+    std::panic::set_hook(Box::new(console_error_panic_hook::hook));
+    wasm_bindgen_futures::spawn_local(run());
+}
+
+#[cfg(target_arch = "wasm32")]
+fn main() {}
+
+#[cfg(not(target_arch = "wasm32"))]
 fn main() {
     env_logger::init();
-
     futures::executor::block_on(run());
 }

src/backend/native.rs

@@ -431,6 +431,20 @@ pub(crate) fn command_encoder_copy_buffer_to_texture(
     );
 }
 
+pub(crate) fn command_encoder_copy_texture_to_buffer(
+    command_encoder: &CommandEncoderId,
+    source: crate::TextureCopyView,
+    destination: crate::BufferCopyView,
+    copy_size: wgt::Extent3d,
+) {
+    wgn::wgpu_command_encoder_copy_texture_to_buffer(
+        *command_encoder,
+        &map_texture_copy_view(source),
+        &map_buffer_copy_view(destination),
+        copy_size,
+    );
+}
+
 pub(crate) fn begin_compute_pass(command_encoder: &CommandEncoderId) -> ComputePassId {
     unsafe { wgn::wgpu_command_encoder_begin_compute_pass(*command_encoder, None) }
 }

src/backend/web.rs

@@ -746,6 +746,19 @@ pub(crate) fn command_encoder_copy_buffer_to_texture(
     );
 }
 
+pub(crate) fn command_encoder_copy_texture_to_buffer(
+    command_encoder: &CommandEncoderId,
+    source: crate::TextureCopyView,
+    destination: crate::BufferCopyView,
+    copy_size: wgt::Extent3d,
+) {
+    command_encoder.copy_texture_to_buffer_with_gpu_extent_3d_dict(
+        &map_texture_copy_view(source),
+        &map_buffer_copy_view(destination),
+        &map_extent_3d(copy_size),
+    );
+}
+
 pub(crate) fn begin_compute_pass(command_encoder: &CommandEncoderId) -> ComputePassId {
     let mapped_desc = web_sys::GpuComputePassDescriptor::new();
     command_encoder.begin_compute_pass_with_descriptor(&mapped_desc)

src/lib.rs

@@ -923,7 +923,6 @@ impl CommandEncoder {
         backend::command_encoder_copy_buffer_to_texture(&self.id, source, destination, copy_size);
     }
 
-    /*
     /// Copy data from a texture to a buffer.
     pub fn copy_texture_to_buffer(
         &mut self,
@@ -931,14 +930,10 @@ impl CommandEncoder {
         destination: BufferCopyView,
         copy_size: Extent3d,
     ) {
-        wgn::wgpu_command_encoder_copy_texture_to_buffer(
-            self.id,
-            &source.into_native(),
-            &destination.into_native(),
-            copy_size,
-        );
+        backend::command_encoder_copy_texture_to_buffer(&self.id, source, destination, copy_size);
     }
 
+    /*
         /// Copy data from one texture to another.
         pub fn copy_texture_to_texture(
             &mut self,