Record frame num in view stats (#2908)

* Add 'frameNumber' to Frame struct

Previously, the frame number returned from bgfx::frame() was tracked separately in the Context. Now,
we store that information in the Frame. This will allow us to attach the frame number to ViewStats.

* Add frame number to ViewStats

When ViewStats are enabled, we tag each timer query with the current frame number, then include
that information in the final results. In this way, clients can correlate specific work that they
submitted to specific GPU timing information.

NOTE: Some backends not implemented, yet. They will always have 0 for frame number.
The ones which are implemented are:
 * OpenGL
 * Vulkan
 * D3D 9,11,12
 * Noop

src/bgfx_p.h

@@ -2076,6 +2076,7 @@ namespace bgfx
 		Frame()
 			: m_waitSubmit(0)
 			, m_waitRender(0)
+			, m_frameNum(0)
 			, m_capture(false)
 		{
 			SortKey term;
@@ -2109,7 +2110,7 @@ namespace bgfx
 			}
 
 			reset();
-			start();
+			start(0);
 			m_textVideoMem = BX_NEW(g_allocator, TextVideoMem);
 		}
 
@@ -2126,12 +2127,12 @@ namespace bgfx
 
 		void reset()
 		{
-			start();
+			start(0);
 			finish();
 			resetFreeHandles();
 		}
 
-		void start()
+		void start(uint32_t frameNum)
 		{
 			m_perfStats.transientVbUsed = m_vboffset;
 			m_perfStats.transientIbUsed = m_iboffset;
@@ -2145,6 +2146,7 @@ namespace bgfx
 			m_cmdPost.start();
 			m_capture = false;
 			m_numScreenShots = 0;
+			m_frameNum = frameNum;
 		}
 
 		void finish()
@@ -2353,6 +2355,8 @@ namespace bgfx
 		int64_t m_waitSubmit;
 		int64_t m_waitRender;
 
+		uint32_t m_frameNum;
+
 		bool m_capture;
 	};
 
@@ -4523,7 +4527,7 @@ namespace bgfx
 			cmdbuf.write(_handle);
 			cmdbuf.write(_data);
 			cmdbuf.write(_mip);
-			return m_frames + 2;
+			return m_submit->m_frameNum + 2;
 		}
 
 		void resizeTexture(TextureHandle _handle, uint16_t _width, uint16_t _height, uint8_t _numMips, uint16_t _numLayers)