feat: ios texture compression to ETC2

[kuruk-mm]

Fix #417

This requires a custom Godot version for iOS which you can get from: https://github.com/dclexplorer/godot.

The custom Godot changes the behaviour of the PortableCompressedTexture2D and enables etcpak compression and decompression for runtime.

Steps to build from source (after you have the XCode project built):

# Clone custom godot
git clone https://github.com/dclexplorer/godot

# Build custom godot
scons p=ios target=template_release

# Copy the result to the XCode solution
IOS_EXPORT_PATH=your_ios_path
cp libgodot.ios.template_release.arm64.a "${IOS_EXPORT_PATH}/Decentraland Mobile Client.ipa.xcodeproj.xcframework/ios-arm64/libgodot.a"
cp libgodot.ios.template_release.arm64.a "${IOS_EXPORT_PATH}/Decentraland Mobile Client.xcframework/ios-arm64/libgodot.a"

Changes to Godot:
dclexplorer/godot@1370898