[Feat] (DirectVulkan): GPU transform feedback capture via VK_EXT_transform_feedback

Second stage of GL 3.0 transform feedback: captured draws now write real
data.

- Device setup enables the VK_EXT_transform_feedback feature when present
  and loads the bind/begin/end entry points.
- Captured draws compile an XfbCapture program variant whose last
  vertex-processing stage gets XfbBuffer/XfbStride/Offset decorations from
  the program's resolved varyings (a new spirv-opt pass). A captured
  gl_Position is mirrored into a dedicated output written before every
  OpReturn - or before every OpEmitVertex in a geometry stage - ahead of
  the position fixup, so the captured value is the shader's own pre-remap
  position.
- DrawArrays/DrawElements wrap the draw in Begin/EndTransformFeedbackEXT;
  a small counter buffer resumes the append position across draws within
  one glBeginTransformFeedback (fresh Begin starts at the bound offsets).
- Capture targets are promoted to persistently-mapped host-coherent GPU
  storage (persistent-map storage now also carries the transform feedback
  usage), so MapBuffer/GetBufferSubData read the captured bytes after the
  fence wait glEndTransformFeedback now performs.
- Draw-mode/feedback-mode validation defers to the geometry shader's
  output primitive when one is present, and glGetBooleanv reports
  GL_TRANSFORM_FEEDBACK_ACTIVE/PAUSED so dEQP's per-case state reset can
  unwind an active capture.

KHR-GL33: transform_feedback capture_vertex_*/capture_geometry_*/
discard_*/draw_xfb and clip_distance.coverage now pass; queries
(PRIMITIVES_WRITTEN) and gl_ClipDistance capture remain.

MobileGL/MG_Backend/DirectVulkan/Renderer/VulkanRenderer.h

@@ -455,6 +455,23 @@ namespace MobileGL::MG_Backend::DirectVulkan {
                                                                  Uint32 stride);
         static inline PFNDrawIndexedIndirectCountFunc s_vkCmdDrawIndexedIndirectCount = nullptr;
 
+        // VK_EXT_transform_feedback (GL transform feedback capture)
+        Bool m_transformFeedbackFeatureEnabled = false;
+        static inline PFN_vkCmdBindTransformFeedbackBuffersEXT s_vkCmdBindTransformFeedbackBuffersEXT = nullptr;
+        static inline PFN_vkCmdBeginTransformFeedbackEXT s_vkCmdBeginTransformFeedbackEXT = nullptr;
+        static inline PFN_vkCmdEndTransformFeedbackEXT s_vkCmdEndTransformFeedbackEXT = nullptr;
+        // Counter buffers (one 4-byte slot per capture binding) let consecutive
+        // draws within one glBeginTransformFeedback append GL-style.
+        VkBufferObject m_xfbCounterBuffer;
+        // Non-zero while inside a GL Begin/End with at least one captured draw
+        // recorded; selects counter-buffer resume on the next captured draw.
+        Bool m_xfbCountersValid = false;
+        Uint64 m_xfbLastSeenGeneration = 0;
+        // Wraps a recorded draw with BeginTransformFeedbackEXT/EndTransformFeedbackEXT
+        // when GL transform feedback is active; binds capture buffers on demand.
+        Bool BeginXfbCaptureForDraw(FrameContext::FrameData& frame);
+        void EndXfbCaptureForDraw(FrameContext::FrameData& frame, Bool began);
+
         VkCommandPool m_commandPool = VK_NULL_HANDLE;
 
         VkBufferManager m_bufferManager;