example-runner-wgpu: add --force-spirv-passthru and rely on it for debugPrintf.

[eddyb]

Not sure yet if this is the way to do it, but I used this for testing the mouse shader (before #1018).

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EDIT: turns out that this is the main requirement for using debugPrintf from wgpu!

Quick demo panic: --- a/examples/shaders/sky-shader/src/lib.rs +++ b/examples/shaders/sky-shader/src/lib.rs @@ -65,2 +65,3 @@ fn sun_intensity(zenith_angle_cos: f32) -> f32 { let cutoff_angle = PI / 1.95; // Earth shadow hack + assert!(zenith_angle_cos > 0.07479); SUN_INTENSITY_FACTOR Enabling debugPrintf output without code changes (nor debug mode): VK_LOADER_LAYERS_ENABLE='VK_LAYER_KHRONOS_validation' \ VK_LAYER_ENABLES='VK_VALIDATION_FEATURE_ENABLE_DEBUG_PRINTF_EXT' \ DEBUG_PRINTF_TO_STDOUT=1 \ cargo run -p example-runner-wgpu --release -- --force-spirv-passthru (also check out the updated DebugPrintfThenExit docs!)