We still do not compile the Vulkan backend for Android by default, this
simply makes it possible to use the samples on Vulkan with a one-line
change, which is useful for testing purposes.
This change also makes it so that the materials used for the samples
include SPIR-V. This makes them fatter but they are merely samples.
I still consider Vulkan on Android to be experimental, there are some
features that need to be implemented.
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