shader_checker: Add support for delivering layer messages via callback
shader_checker: Add support for delivering layer messages via callback

This is just a direct copy of what all the other layers are doing.
Allows the layer validation tests to capture errors.

Signed-off-by: Chris Forbes <5dcac483e7a6e31a5cbe89f38cc69dff8ee93dac@ijw.co.nz>

KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools,KhronosGroup/Vulkan-Tools