[Vk] Fix assert in complex conditions

VulkanRenderSystem::flushBoundGpuProgramParameters would assert inside
constBuffer->regressFrame();

The steps to trigger are:

1. Use mAutoParamsBuffer (i.e. a low level material with parameters) so
that mFirstUnflushedAutoParamsBuffer becomes non-zero.
2. Stall twice in a row without presenting to screen.

In real world scenarios this could happen if a lot of texture memory is
streaming and TextureGpuManager is forced to stall.

RenderSystems/Vulkan/include/OgreVulkanRenderSystem.h

@@ -272,7 +272,6 @@ namespace Ogre
                                        uint16 variabilityMask ) override;
         void bindGpuProgramPassIterationParameters( GpuProgramType gptype ) override;
 
-    protected:
         /** Low Level Materials use a params buffer to pass all uniforms. We emulate this using a large
             const buffer to which we write to and bind the regions we need.
             This is done in bindGpuProgramParameters().

RenderSystems/Vulkan/src/OgreVulkanDevice.cpp

@@ -678,6 +678,20 @@ namespace Ogre
     //-------------------------------------------------------------------------
     void VulkanDevice::stall()
     {
+        // We must call flushBoundGpuProgramParameters() now because commitAndNextCommandBuffer() will
+        // call flushBoundGpuProgramParameters( SubmissionType::FlushOnly ).
+        //
+        // Unfortunately that's not enough because that assumes VaoManager::mFrameCount
+        // won't change (which normally wouldn't w/ FlushOnly). However our caller is
+        // about to do mFrameCount += mDynamicBufferMultiplier.
+        //
+        // The solution is to tell flushBoundGpuProgramParameters() we're changing frame idx.
+        // Calling it again becomes a no-op since mFirstUnflushedAutoParamsBuffer becomes 0.
+        //
+        // We also *want* mCurrentAutoParamsBufferPtr to become nullptr since it can be reused from
+        // scratch. Because we're stalling, it is safe to do so.
+        mRenderSystem->flushBoundGpuProgramParameters( SubmissionType::NewFrameIdx );
+
         // We must flush the cmd buffer and our bindings because we take the
         // moment to delete all delayed buffers and API handles after a stall.
         //