Encode alpha for Images

[DJMcNab]

Currently, when encoding an image, the alpha factor is discarded:

vello/vello_encoding/src/encoding.rs

Lines 420 to 424 in 4433203

	pub fn encode_image(&mut self, image: &Image, alpha: f32) {
	let _alpha = alpha * f32::from(image.alpha);
	// TODO: feed the alpha multiplier through the full pipeline for consistency
	// with other brushes?
	// Tracked in https://github.com/linebender/vello/issues/692

The consequences of this are:

• DrawGlyphs::brush_alpha doesn't work for bitmap glyphs
• The alpha factor on Peniko's Image type is not handled - Rename with_alpha_factor to multiply_alpha peniko#52

To fix this, we need to wire this alpha through to fine (because of #664 and friends), somehow.