It now automatically sets the size to the current project's size, has a button to lock aspect ratio, and resizing based on percentage. Some UI changes to CreateNewImage too.
Fixes #377. The issue was that the shader material was being shared with the background of the cel buttons, and when the cel button transparent background was resized, the canvas transparent background was also affected.
This will be the parent of all (or most) image effect dialog nodes. Currently only parent on FlipImageDialog, will change to be parent of the rest of the effects.
This comes with the benefit that we no longer need a Viewport node in the ShaderEffect dialog, which lets us have the Preview TextureRect at a fixed size. Previously, the preview was scaled based on the image's size, which would make applying shaders to large images very inconvenient, since the dialog would be way too big.
Not sure if options for RGBA channels are really needed here, but I kept them from invert colors. We could remove them in the future if they are unneeded.
