Add image mark

[jonmmease]

Adds support for image marks. Some notes:

• The scenegraph works with in-memory images. Loading images from URLs is handled in the vega logic using reqwest.
• Images are written to a texture atlas, positioned using etagere. The texture coordinates are stored in the vertex structs.
• If there are too many images to fit in one texture, the texture us updated once per batch.
• Following the design of other marks, I added a generic TextureRenderer that is parameterized by a TextureRenderShader. The image shader contains the image atlas logic. I haven't looked deeply into gradiants, but it's possible there will be a separate GradientShader, or perhaps gradients will reuse the image shader.
• aspect, align, baseline, and smooth are all supported

Here are some examples rendered by this PR:

Vega-Lite logos example:

aspect = false

aspect = true with baseline and align specifications

expanded image with smooth=true

expanded image with smooth=false

example with larger images that triggers multi-batch logic: