Fixed several crash/mem leak issue related to multiple device init/shutdown.

src/renderer_mtl.mm

@@ -432,6 +432,7 @@ namespace bgfx { namespace mtl
 				return false;
 			}
 
+			retain(m_device);
 			m_metalLayer.device      = m_device;
 			m_metalLayer.pixelFormat = MTLPixelFormatBGRA8Unorm;
 
@@ -488,6 +489,7 @@ namespace bgfx { namespace mtl
 			}
 
 			m_screenshotBlitProgram.create(&m_screenshotBlitProgramVsh, &m_screenshotBlitProgramFsh);
+			release(lib);
 
 			reset(m_renderPipelineDescriptor);
 			m_renderPipelineDescriptor.colorAttachments[0].pixelFormat = m_metalLayer.pixelFormat;
@@ -672,6 +674,11 @@ namespace bgfx { namespace mtl
 				m_textures[ii].destroy();
 			}
 
+			m_screenshotBlitProgramVsh.destroy();
+			m_screenshotBlitProgramFsh.destroy();
+			m_screenshotBlitProgram.destroy();
+			MTL_RELEASE(m_screenshotBlitRenderPipelineState);
+
 			captureFinish();
 
 			MTL_RELEASE(m_depthStencilDescriptor);
@@ -683,10 +690,7 @@ namespace bgfx { namespace mtl
 			MTL_RELEASE(m_samplerDescriptor);
 
 			MTL_RELEASE(m_backBufferDepth);
-			if (BX_ENABLED(BX_PLATFORM_IOS) )
-			{
-				MTL_RELEASE(m_backBufferStencil);
-			}
+			MTL_RELEASE(m_backBufferStencil);
 
 			for (uint8_t i=0; i < MTL_MAX_FRAMES_IN_FLIGHT; ++i)
 			{
@@ -1170,11 +1174,18 @@ namespace bgfx { namespace mtl
 				{
 					release(m_backBufferDepth);
 				}
+
 				m_backBufferDepth   = m_device.newTextureWithDescriptor(m_textureDescriptor);
 
+				if (NULL != m_backBufferStencil)
+				{
+					release(m_backBufferStencil);
+				}
+
 				if (m_hasPixelFormatDepth32Float_Stencil8)
 				{
 					m_backBufferStencil = m_backBufferDepth;
+					retain(m_backBufferStencil);
 				}
 				else
 				{
@@ -1915,6 +1926,7 @@ namespace bgfx { namespace mtl
 		if (NULL != lib)
 		{
 			m_function = lib.newFunctionWithName(SHADER_FUNCTION_NAME);
+			release(lib);
 		}
 
 		BGFX_FATAL(NULL != m_function
@@ -2831,6 +2843,7 @@ namespace bgfx { namespace mtl
 
 	void CommandQueueMtl::shutdown()
 	{
+		finish(true);
 		MTL_RELEASE(m_commandQueue);
 	}