Add file image preview for Graph Editor

[kwokcb]

Feature Update

For better user feedback on what images are being used from the UI, this change adds image previews for file image nodes

Details:

• Adds a per filename input image preview
• Default size is 256 px but is settable from the command line using the --previewWidth argument. A size of <= 0 will turn off previews.
• Preview size will resize to fit up to the maximum property editor panel width if desired preview size is greater than the editor width.

Implementation notes

• Adds fix to use document "resolved" image path otherwise relative path images may not be found (e.g. std surface brick)

Caveats

• There is no software (C++) color management support to display images in the input color space.

Examples

• Image node (resizing)

Resized to fit	Full preview size

2nd Image node	Usd Texture

gltf Texture	tri-planar

[kwokcb]

This is another port from my editor. For now there is no way to support colorspace transforms via C++ thus images do not consider input space. Let me know what you think. Thanks.

[jstone-lucasfilm]

This looks like a great contribution, thanks @kwokcb, and I'm CC'ing @lfl-eholthouser for a more formal review!