Add an Image Frame node and make the graph output vector data

[Keavon]

Now that images are nodes, the next highest priority item is making bitmap data into vector data with an Image Frame node (that creates an <image /> tag), and making the whole node graph export vector data rather than bitmap data. (Only one vector element can output for now.)

The graph's Input node has a secondary output value called "Transform" which exposes the value of the layer's Transform. Here's what the default graphs look like for an Image and an Imaginate:

The Transform input to the Imaginate node, if hooked up, allows the node to automatically pick its image generation dimensions (by rounding the Transform input's scale to the nearest 64px). It is also used to set the transform of the frame used in rasterization of a referenced mask layer.

If the Transform is not provided, the user must enter the image generation Dimensions field (its Edit checkbox becomes disabled so the W and H fields are set to editable regardless of that checkbox state). We should also invert that checkbox so instead of "enable the checkbox to manually edit", it's "enable the checkbox to to use the auto value". So "auto" is the default (checked), opposite of how it is now. I just need to find an icon that conveys "auto" instead of "edit" (the current ✏️ icon).