Check and enable VK_KHR_sampler_mirror_clamp_to_edge

Check and enable VK_KHR_sampler_mirror_clamp_to_edge

Several tests in dEQP-VK.pipeline group assume that
VK_SAMPLER_ADDRESS_MODE_MIRROR_CLAMP_TO_EDGE mode is supported. Spec
actually requires that VK_KHR_sampler_mirror_clamp_to_edge is enabled if
that mode is used.

This change enables VK_KHR_sampler_mirror_clamp_to_edge in default
device, if supported, and adds relevant checks in the test code.

In the future we'll want to clean this up by giving test cases more
control about the device configuration they need.

Fixes #339

Change-Id: Ib264c4f2fb147271c9c09d6286f4a26dd175f493