add ability to load .dds, .tga, and .jpeg texture formats

[blunted2night]

controled by non-defaulted features of the same name

[cart]

Two small notes:

• In general I would prefer to not use macros in cases like this. It might involve a bit of extra code duplication, but it improves debuggability / legibility.
• In this case I think the macro conversation isn't super important because this seems like the right call:

const FILE_EXTENSIONS: &[&str] = &["png", "dds", "tga", "jpg", "jpeg"];

impl AssetLoader for ImageTextureLoader {
    fn load<'a>(
        &'a self,
        bytes: &'a [u8],
        load_context: &'a mut LoadContext,
    ) -> BoxedFuture<'a, Result<()>> {
        Box::pin(async move {
            use bevy_core::AsBytes;

            // Find the image type we expect. A file with the extension "png" should
            // probably load as a PNG.

            let ext = load_context.path().extension().unwrap().to_str().unwrap();

            let img_format = image::ImageFormat::from_extension(ext)
                .ok_or_else(|| {
                    format!(
                    "Unexpected image format {:?} for file {}, this is an error in `bevy_render`.",
                    ext,
                    load_context.path().display()
                )
                })
                .unwrap();

[blunted2night]

That is much better, I updated the pull request.