Due to an assertion in etc2comp, this unfortunately uses RGB8 instead of
RG11 but it still helps quite a bit (3.3 MB savings in the unzipped KTX
size).
This shrinks file size as follows, although it seems to darken the
rendered result.
```
1398209 Oct 2 16:22 ao.ktx
699172 Oct 2 16:20 ao_etc.ktx
1398209 Oct 2 16:22 metallic.ktx
699172 Oct 2 16:19 metallic_etc.ktx
1398209 Oct 2 16:22 roughness.ktx
699172 Oct 2 16:20 roughness_etc.ktx
```
At build time, mipgen now creates several compressed and non-compressed
variants for albedo.
For ASTC we use the ARM encoder, for S3TC we use the STB encoder. The
latter is somewhat limited so we may wish to investigate other libraries
in the future (e.g., AMD Compressenator).
At run time, we detect which of these variants to download, based on
available WebGL extensions. In practice, this means that desktop
browsers will use the S3TC variant and mobile browsers will use the ASTC
variant.
The makes unzipped albedo go from ~4 MB to 682 KB using S3TC.
Mobile support (ASTC and ETC2) is coming in a future PR, stay tuned.
The compression API now takes block size instead of bit rate. Otherwise
it is quite possible for the encoder to generate a block size that is
not one of the valid block sizes that graphics API's can actually
consume.
This was designed as a standalone tool rather than a library, so we are
renaming their "main" function to "standalone_main", which is described
in the tnt README.
We'll also be adding a wrapper to our imageio library.
