MakeTiles: refactor: load images at parse time

this will allow me to detect and load animations (in a future commit)
MakeTiles: use constant (STD_SIZE) to represent standard tile size of 16px
MakeTiles: make tile drawing code more object-orientated

Separate classes for tile layers, tile sprites, etc., to make it easier
to introduce more complicated/flexible tile configurations in the future.
MakeTiles: implement support for files specifying an animation
MakeTiles: track target size when loading a source image
MakeTiles: refactor: paving the way to allow multiple images per tile
MakeTiles: working on support for animated tiles
MakeTiles: fix a couple bugs in code for loading animation
MakeTiles: refactor: move TileImage classes into separate file
MakeTiles: Animation: draw the frames for different times during the animation
TileImages: put placeholder for animated tile support
TileImages: use experimental "fountain.ini" animation file