Added separate CMAKE options to include each of the individual examples as well as one global option to include all examples.
I set build all examples to OFF as a default since downloading all of SDL 2 and 3 seems excessive. Instead, the demo from the video and the website are turned on by default.
A note about these options should probably be added to the ReadMe as well, but I wasn't sure where the logical place to put it would be.
Makes including the examples optional. SDL2/SDL3 specifically have long download/build times. If they aren't being used, then this can dramatically speed up build times.
