Improvements to the build process

[kring]

• Upgrade KTX-Software to v4.3.2. See https://github.com/CesiumGS/KTX-Software/pull/1.
• Use the ktx library instead of ktx_read. While cesium-native currently only needs the read functionality, by depending on ktx_read cesium-native prevents anything that uses cesium-native from bringing in the write functionality. This is because there's no ktx_write. The only options are ktx_read which is read-only, and ktx which includes both. If you try to link against both ktx and ktx_read, you'll end up with multiply-defined symbols.
• Move the STB image resize implementation code into CesiumGltfReader (instead of CesiumGltfContent). This fixes a weird situation where applications that only depend on CesiumGltfReader would get linker errors.
• Disable compilation of the KTX CLI by setting KTX_FEATURE_TOOLS to off. This saves a bit of time, and also stops KTX from bringing in its only implementation of cxxopts.

[lilleyse]

• Move the STB image resize implementation code into CesiumGltfReader (instead of CesiumGltfContent). This fixes a weird situation where applications that only depend on CesiumGltfReader would get linker errors.

Does this fix https://github.com/CesiumGS/cesium-native/issues/813?

[lilleyse]

Looks good!