vkd3d/command: Keep track of count of SO buffers.
Fixes validation layer issue VUID 09630 with vkCmdBeginTransformFeedbackEXT: "The Vulkan spec states: The sum of firstCounterBuffer and counterBufferCount must be less than or equal to the number of transform feedback buffers currently bound by vkCmdBindTransformFeedbackBuffersEXT".
Found while poking at llvmpipe test failures.