This is useful for unit tests. The content of the swapchain can be
read to main memory with Renderer::readPixels().
This PR doesn't implement this new feature in the following backends and
platforms (TODO):
VulkanDriver.cpp
MetalDriver.mm
PlatformWGL.cpp
PlatformWebGL.cpp
PlatformCocoaTouchGL.mm
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.
