add is_valid, get_i_min, get_i_max, get_j_min, get_j_max to API

[Sid-Bhatia-0]

1. add is_valid, get_i_min, get_i_max, get_j_min, get_j_max to the official API. Add methods for shapes that did not have them.
2. For Bitmap, Character, TextLine, if the bitmap is empty or if characters are non-printable, then i_min == i_max and j_min == j_max
3. Fix bug in get_i_max for ThickLine.
4. Use the general convert(I, 2) instead of just 2.