images: Report total image data size
screen: API for VRAM allocation
game: add #define for shield layer index
screen: setup_tile_layer() takes palette and offset args
screen: update get_image_offset() function
screen: Set contents of more than one palette

Chronocube supports four palettes, after all.
screen: API to set individual palette entry
screen: Add color key arg to tile layer setup
screen: Set sprites to above highest tile layer
Add starfield generator

This generates a random starfield of a given size, density, and
intensity range.  Each starfield is generated as a tile layer.
game: Add two starfields

Using the new starfield generator, the game gets a facelift with
starfields.