Fix pvrtc encoder

[DrMoriarty]

Always resize image to square of power2
Enable mipmaps only if original texture has it enabled
Fix #28534, fix #28541

[akien-mga]

Changes seem good, just need a quick check from @reduz to confirm if there was any reason to modulate by 8 instead of po2.

[akien-mga]

Comments from @reduz on IRC:

13:58 <reduz> Akien: well, technically po2 is only needed when you use mipmaps on GLES2
14:15 <@Akien> reduz: so they should keep the previous logic with % 8 when there's no mipmaps I guess?
14:15 <reduz> Akien: i think so, but the importer I think already resizes to po2
14:15 <reduz> Akien: so i am not quite sure what the problem is

[DrMoriarty]

@akien-mga may be somebody should fix thirdparty pvrtc encoder?
file godot3/thirdparty/pvrtccompressor/PvrTcEncoder.cpp:

 267|void PvrTcEncoder::EncodeRgb4Bpp(void* result, const RgbaBitmap& bitmap)
 268|{
 269|    assert(bitmap.GetBitmapWidth() == bitmap.GetBitmapHeight());
 270|    assert(BitUtility::IsPowerOf2(bitmap.GetBitmapWidth()));

[DrMoriarty]

@akien-mga I'm looking around the Internet and can not find open pvrtc encoder which doesn't require square pow2 texture input.

[akien-mga]

Could not get another answer from @reduz, so let's stick to what the thirdparty library requires. Thanks for looking into this :)

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.1.2.