Don't decode grayscale PNGs to RGB

[mbrubeck]

servo/rust-png#45 makes our PNG library always decode to RGBA format in memory, because our graphics code (azure) doesn't work with grayscale sources. We could save memory for grayscale images by adding support for grayscale in-memory representations.

[atouchet]

Servo no longer uses Azure. Is this still an issue?

[jdm]

According to

servo/components/net_traits/image/base.rs

Lines 53 to 67 in 292d6ec

	Ok(_) => match piston_image::load_from_memory(buffer) {
	Ok(image) => {
	let mut rgba = match image {
	DynamicImage::ImageRgba8(rgba) => rgba,
	image => image.to_rgba(),
	};
	pixels::rgba8_byte_swap_colors_inplace(&mut *rgba);
	Some(Image {
	width: rgba.width(),
	height: rgba.height(),
	format: PixelFormat::BGRA8,
	bytes: IpcSharedMemory::from_bytes(&*rgba),
	id: None,
	cors_status,
	})

it appears yes.