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.

android/filament-utils-android/src/main/cpp/Utils.cpp

@@ -99,8 +99,8 @@ JNIEXPORT jint JNI_OnLoad(JavaVM* vm, void*) {
 
     int rc;
 
-    // KTXLoader
-    jclass ktxloaderClass = env->FindClass("com/google/android/filament/utils/KTXLoader");
+    // KTX1Loader
+    jclass ktxloaderClass = env->FindClass("com/google/android/filament/utils/KTX1Loader");
     if (ktxloaderClass == nullptr) return JNI_ERR;
     static const JNINativeMethod ktxMethods[] = {
         {(char*)"nCreateKTXTexture", (char*)"(JLjava/nio/Buffer;IZ)J", reinterpret_cast<void*>(nCreateKTXTexture)},