Add the `unpackI16B_I16` unpacker to unpackers

[FirefoxMetzger]

I would like to propose to add a new unpacker to Unpack.c that can unpack big-endian 16-bit integer pixels (I;16B) into 16-bit integer pixels (I;16, I assume native or does it mean little-endian?).

At the moment, the following conversions are supported:

Pillow/src/libImaging/Unpack.c

Lines 1761 to 1773 in 139acc4

	/* storage modes */
	{"I;16", "I;16", 16, copy2},
	{"I;16B", "I;16B", 16, copy2},
	{"I;16L", "I;16L", 16, copy2},
	{"I;16N", "I;16N", 16, copy2},
	{"I;16", "I;16N", 16, unpackI16N_I16}, // LibTiff native->image endian.
	{"I;16L", "I;16N", 16, unpackI16N_I16}, // LibTiff native->image endian.
	{"I;16B", "I;16N", 16, unpackI16N_I16B},
	{"I;16", "I;16R", 16, unpackI16R_I16},
	{"I;16", "I;12", 12, unpackI12_I16}, // 12 bit Tiffs stored in 16bits.

and it would be really useful for me to also have I;16B->I;16 as well.

My use case is 16-bit PNGs, which I could then directly unpack into a 16-bit buffer. Currently, we can only unpack them into a 32-bit (I) buffer or a 16-bit big-endian buffer (I;16B), so I have to copy the data one more time from that buffer into I;16. With this small addition I could avoid/save this copy :)

[radarhere]

I've created PR #7125 to resolve this.

[FirefoxMetzger]

Awesome! Thanks for this @radarhere 🚀