Support all reasonable cursor sizes. Reduce the size of the standard
cursor from 16x16 (with 6 columns unused) to 10x16 and rename it to
the "small" cursor. Add a "large" 19x32 cursor and use it for screen
widths larger than 800 pixels. Use libvgl's too-small indentation for
the large data declarations.

MOUSE_IMG_SIZE = 16 is still part of the API. If an application supplies
invalid bitmaps for the cursor, then the results may be different from
before.