Support all reasonable cursor sizes. Reduce the size of the standard

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.