swscale: Disallow conversion to GBRP16

This reverts parts of d6d5ef5534d582, that didn't work right. (The
tests that were added failed on big endian, and the output looked
garbled on little endian as well.)

This is due to the fact that the intermediate scaling values (from
e.g. hScale8To19_c or hScale16To19_c) are stored as int32_t and
thus requires a separate output function, while yuv2gbrp_full_X_c
only interprets it as int16_t.

Signed-off-by: Martin Storsjö <martin@martin.st>