UI updates and preroll buffer to 8 frames

apps/LoopThroughWithOpenGLCompositing/OpenGLComposite.cpp

@@ -63,6 +63,7 @@ constexpr GLuint kDecodedVideoTextureUnit = 1;
 constexpr GLuint kSourceHistoryTextureUnitBase = 2;
 constexpr GLuint kPackedVideoTextureUnit = 2;
 constexpr GLuint kGlobalParamsBindingPoint = 0;
+constexpr unsigned kPrerollFrameCount = 8;
 const char* kVertexShaderSource =
 	"#version 430 core\n"
 	"out vec2 vTexCoord;\n"
@@ -1233,7 +1234,7 @@ bool OpenGLComposite::Start()
 	mTotalPlayoutFrames = 0;
 
 	// Preroll frames
-	for (unsigned i = 0; i < 5; i++)
+	for (unsigned i = 0; i < kPrerollFrameCount; i++)
 	{
 		// Take each video frame from the front of the queue and move it to the back
 		IDeckLinkMutableVideoFrame* outputVideoFrame = mDLOutputVideoFrameQueue.front();