Implement the Iterator trait for Image

## What problem does this solve or what need does it fill?

If a texture has it's transparent areas encoded as a mock color. It'd be nice to iterate through the pixels to replace the mock color with transparency

## What solution would you like?

bevy::prelude Struct Image should implement the Iterator trait

## What alternative(s) have you considered?

Right now to iterate through all the pixels of a texture you have to make a for loop with 2 indexes. Iterator doesn't have any overhead and is easier to read and write.

## Additional context

Imagine the green is the mock color and is supposed to be replaced with a 0 alpha value (absolute transparency):