#1969 - Give graphics events the ability to override the standard process

While graphics_rotate, graphics_resize, and graphics_composite events already exist, they don't have the ability to *override* the standard process.
This makes it a bit tricky when you want to replace the standard procedure with another (e.g. use jpegtran to perform lossless jpg rotation).
Solution:
- make a temp filename.
- tell the events to use it as the output file.
- if an event makes something, use it and skip the standard process.