Enable writing ETC2

[Trimatix]

Notes for reviewer:

• there's no immediately obvious ETC2 codec to use, except for etcpak. etcpak's ETC2 compression function returns almost completely black images, it doesn't seem to be working properly.
  This PR is a stopgap solution - ETC2 is backwards compatible with ETC1, so for now we can just compress with ETC1.
  AEIs compressed in this way are opening as expected using tex2img.

[github-actions]

Coverage Report

File	Stmts	Miss	Cover	Missing
src/AEPi
codec.py	37	3	92%	22, 40, 119
constants.py	84	7	92%	38–44, 64, 70
exceptions.py	40	11	72%	18–19, 28–29, 39–40, 47, 54, 61, 68–69
src/AEPi/codecs
EtcPakCodec.py	30	16	47%	12–13, 24–38, 43–49
RawCodec.py	13	2	85%	15, 20
Tex2ImgCodec.py	24	5	79%	12–13, 32, 38–39
__init__.py	8	2	75%	11–12
src/AEPi/image
AEI.py	241	27	89%	121, 181, 185, 198, 225, 288, 294–297, 302, 326, 333, 341–342, 347–351, 382, 396–397, 436–437, 444–445, 488–489
texture.py	19	2	89%	29, 49
src/AEPi/lib
binaryio.py	24	1	96%	64
imageOps.py	9	2	78%	13–14
src/tests/image
test_AEI.py	199	1	99%	29
TOTAL	850	79	91%

Tests	Skipped	Failures	Errors	Time
43	0 💤	0 ❌	0 🔥	0.349s ⏱️