Document border handling on rotation

[gfiumara]

Investigate—and likely change—the border handling of invalid regions when rotating a block.

A region of size 32x16 is only fully contained inside a region of size 32x32 for 4 distinct angles – how to deal with all other angles?

Are invalid regions cropped, as specified in the standard?

Source: Comment DE-071 from ISO/IEC JTC 1/SC 37/WG 3 N 1416 (Clauses 5.2.3.1, 5.2.4.3)

[gfiumara]

Before the line in question:

After the line in question:

Note: both images are 36x36.