Create pixel-only textures as RGB (no alpha). This fixes some issues
with DrawGraph functions writing to an alpha channel that shouldn't
exist (e.g. causes black screen in Haru Urare).
This doesn't fix the analogous issue for CGs which shouldn't have an
alpha channel.
Ensure that sact_get_sprite always returns a sprite object if sprite
number is not negative. SACT2 functions in general should work before
pixel data is attached to a sprite.
This also fixes the default for the 'show' property (true) and changes
the logic for adding/removing sprites from the scene to be less
error-prone (checked whenever a sprite is dirtied).
Split the code from sact_graphics.c into sprite.c and hll/SACT2.c.
sprite.c contains an index-independent sprite/scene interface, whereas
hll/SACT2.c handles the indexing of sprites.
This is to better support the StoatSpriteEngine.MultiSprite_* functions,
which use a different indexing system.
Nunuhara Cabbage