Fix references to images in proposal documents #923

Job	Run time
Run Vulkan CTS framework tests to validate against XML changes	1m 48s
Verify repository license compliance	38s
Build the core-only spec, to try and catch ifdef errors in extension markup	51s
Build the vulkan specification and generate any associated files (such as vulkan.h)	6m 48s
Run various checker scripts on the spec and XML sources	49s
Build a test spec and check against expectation, to make sure the spec toolchain works as expected	1m 20s
Generate the vulkan C++ header (vulkan.hpp)	1m 10s
Compile a simple test program that uses vulkan.h	7s
Generate Rust bindings (Ash crate)	47s
Build-test Rust bindings (Ash crate)	25s
Compile a simple test program that uses vulkan.hpp	1m 27s
	16m 10s

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