SwiftShader emits warnings (and so fails tests) due to subgroup size control

https://github.com/google/amber/pull/913 failed presubmits because it rolls SwiftShader forward, and SwiftShader now emits a waning to stdout about `VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SUBGROUP_SIZE_CONTROL_FEATURES_EXT` structures being passed to `vkGetPhysicalDeviceFeatures2()` when the `VK_EXT_subgroup_size_control` extension is not supported.
This should be ignored by drivers, but the stdout message [is treated as a test failure](https://github.com/google/amber/blob/0eee2d45d053dfc566baa58442a9b1b708e4f2a7/tests/run_tests.py#L241).

To work around this for the time being, I've landed a change into SwiftShader that can adjust the logging level. We can use this to raise the logging level to only errors and fatal messages.

We might want to consider only passing `VK_STRUCTURE_TYPE_PHYSICAL_DEVICE_SUBGROUP_SIZE_CONTROL_FEATURES_EXT` queries if `VK_EXT_subgroup_size_control` is actually reported as supported by the driver.

See [b/169966461](https://issuetracker.google.com/issues/169966461) for more context.