Add Ktx2Reader and BasisEncoder and use them in samples.

mipgen can now emit basis-encoded KTX2 files. Both the desktop and
web "suzanne" samples use this as a test for compressed textures.

This PR does not add KTX2 support to glTF, but it's on the way.

`BasisEncoder` has a builder style API that calls the basis encoder to
create KTX2 files. This hides some low-level BasisU features that we are
not using, like file I/O and mipmap generation.

`Ktx2Reader` is an easy-to-use API for creating Filament textures from
KTX2 files. Its API primarily consists of these two methods:

    bool requestFormat(Texture::InternalFormat format);
    Filament::Texture* load(const uint8_t* data, size_t size);

The first method is used to build an ordered list of formats that are
supported by your hardware. The second method consumes the contents of a
basis-encoded KTX2 file and attempts to produce a Filament texture with
a preferred format.

IMPORTANT: Our tools still let you use KTX1 for non-compressed images
because it is useful for HDR, but you can no longer use KTX1 for
block-compressed data.

Partial fix for #4771.

libs/image/src/ImageSampler.cpp

@@ -335,6 +335,7 @@ void computeSingleSample(const LinearImage& source, float x, float y, SingleSamp
     }
 }
 
+// Generates the given number of mipmaps (not including the base level) using the given filter.
 // Unlike traditional mipmap generation, our implementation generates all levels from the original
 // image, under the premise that this produces a higher quality result.
 void generateMipmaps(const LinearImage& source, Filter filter, LinearImage* result, uint32_t mips) {