Refactor ShaderDebugTrace to not return a single list of complete states

* The ShaderDebugTrace now only sets up the initial state of an opaque
  ShaderDebugger handle.
* This handle can then be passed to a new function - ContinueDebug - to
  iteratively return N more states. The number of states is implementation
  defined and may be a fixed number or it may run for a fixed time.
* The states themselves no longer contain a complete snapshot of all variables,
  but instead only the changed variables for that iteration. The changes are
  stored as before and after value to make it easier to step forwards and
  backwards (only the ShaderDebugState is needed to move forward or backwards,
  you don't have to search back for the last set value of a variable to 'undo' a
  change).

qrenderdoc/Windows/TextureViewer.cpp

@@ -3758,7 +3758,7 @@ void TextureViewer::on_debugPixelContext_clicked()
   m_Ctx.Replay().AsyncInvoke([this, &trace, &done, x, y](IReplayController *r) {
     trace = r->DebugPixel((uint32_t)x, (uint32_t)y, m_TexDisplay.subresource.sample, ~0U);
 
-    if(trace->states.isEmpty())
+    if(trace->debugger == NULL)
     {
       r->FreeTrace(trace);
       trace = NULL;