Make proposal images work in github repository rendering #920

Job	Run time
Run Vulkan CTS framework tests to validate against XML changes	1m 46s
Build the core-only spec, to try and catch ifdef errors in extension markup	51s
Verify repository license compliance	36s
Run various checker scripts on the spec and XML sources	53s
Build a test spec and check against expectation, to make sure the spec toolchain works as expected	1m 20s
Build the vulkan specification and generate any associated files (such as vulkan.h)	6m 55s
Generate the vulkan C++ header (vulkan.hpp)	1m 2s
Compile a simple test program that uses vulkan.h	12s
Generate Rust bindings (Ash crate)	54s
Build-test Rust bindings (Ash crate)	1m 10s
Compile a simple test program that uses vulkan.hpp	18s
	15m 57s

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