[Fix] (DirectVulkan): make transform feedback writes visible to what reads them

GL makes transform feedback results visible to every later command on their own,
with no glMemoryBarrier in between -- unlike shader storage writes. An
application replaying a capture with glDrawTransformFeedback is therefore
entitled to the captured bytes without asking for them, so the barrier the Vulkan
memory model requires has to come from here.

It cannot be recorded where the write happens: the capturing draw runs inside a
render pass that declares no self-dependency. Flag it there instead and emit the
barrier at the next point that could read the buffer -- the following draw's
setup, or a readback -- ending the render pass first, the same shape
glMemoryBarrier already uses.

The destination covers every way a captured buffer comes back: replayed as vertex
attributes or indices, read through a uniform or storage binding, sourced as an
indirect command, copied out, or mapped.

MobileGL/MG_Backend/DirectVulkan/Renderer/VulkanRenderer.h

@@ -518,6 +518,11 @@ namespace MobileGL::MG_Backend::DirectVulkan {
         // when GL transform feedback is active; binds capture buffers on demand.
         Bool BeginXfbCaptureForDraw(FrameContext::FrameData& frame);
         void EndXfbCaptureForDraw(FrameContext::FrameData& frame, Bool began);
+        // Makes the captured bytes visible to whatever reads them next. Deferred rather than
+        // recorded next to the capture, because the capturing draw runs inside a render pass
+        // that declares no self-dependency.
+        void MakeXfbWritesVisible();
+        Bool m_xfbWritesPendingVisibility = false;
         // Wrap one app draw in an occlusion-query slot while a GL_SAMPLES_PASSED
         // query is active. Returns whether a slot was begun (End must mirror it).
         Bool BeginOcclusionForDraw(VkCommandBuffer commandBuffer);