Add cvars to optimize or shuffle layers

There's plenty of optimization possible when drawing UI primitives where
primitives related by texture or colour can be merged into composite GPU
draw calls. This optimization can currently not be enabled wholesale as
much of the Lua code has hardcoded assumptions about draw order within
layers being preserved.

In order to more clearly manifest such order dependencies there's also
an entertaining option to completely shuffle the draw order of
primitives within each layer.

These new variables are `r_layerOptimize` and `r_layerShuffle`, both
defaulting to 0 (off).

Author: zao

engine/render/r_main.cpp

@@ -9,6 +9,8 @@
 
 #include <fmt/chrono.h>
 #include <map>
+#include <numeric>
+#include <random>
 #include <sstream>
 #include <vector>
 
@@ -297,6 +299,9 @@ void Batch::Execute()
 
 void r_layer_c::Render()
 {
+	bool const optimize = renderer->r_layerOptimize->intVal == 1;
+	bool const shuffle = renderer->r_layerShuffle->intVal == 1;
+
 	struct BatchKey {
 		r_viewport_s viewport = { -1, -1, -1, -1 };
 		int blendMode = -1;
@@ -356,7 +361,7 @@ void r_layer_c::Render()
 		case r_layerCmd_s::QUAD: {
 			Batch* batch{};
 			auto I = batchIndices.find(currentKey);
-			if (I != batchIndices.end()) {
+			if (I != batchIndices.end() && optimize) {
 				batch = &batches[I->second];
 			}
 			else {
@@ -392,9 +397,20 @@ void r_layer_c::Render()
 
 	std::optional<BatchKey> lastKey{};
 	int const numBatches = batches.size();
+
+	std::vector<size_t> batchPermutation(numBatches);
+	{
+		static std::random_device rd;
+		static std::mt19937 g(rd());
+		std::iota(batchPermutation.begin(), batchPermutation.end(), 0);
+		if (shuffle) {
+			std::shuffle(batchPermutation.begin(), batchPermutation.end(), g);
+		}
+	}
+
 	for (int i = 0; i < numBatches; ++i) {
-		auto& batch = batches[i];
-		auto& key = batchKeys[i];
+		auto& batch = batches[batchPermutation[i]];
+		auto& key = batchKeys[batchPermutation[i]];
 		if (!lastKey || lastKey->viewport.x != key.viewport.x || lastKey->viewport.y != key.viewport.y ||
 			lastKey->viewport.width != key.viewport.width	|| lastKey->viewport.height != key.viewport.height)
 		{
@@ -467,6 +483,8 @@ r_renderer_c::r_renderer_c(sys_IMain* sysHnd)
 	r_compress = sys->con->Cvar_Add("r_compress", CV_ARCHIVE, "0");
 	r_screenshotFormat = sys->con->Cvar_Add("r_screenshotFormat", CV_ARCHIVE, "jpg");
 	r_layerDebug = sys->con->Cvar_Add("r_layerDebug", CV_ARCHIVE, "0");
+	r_layerOptimize = sys->con->Cvar_Add("r_layerOptimize", CV_ARCHIVE | CV_CLAMP, "0", 0, 1);
+	r_layerShuffle = sys->con->Cvar_Add("r_layerShuffle", CV_ARCHIVE | CV_CLAMP, "0", 0, 1);
 
 	Cmd_Add("screenshot", 0, "[<format>]", this, &r_renderer_c::C_Screenshot);
 }

engine/render/r_main.h

@@ -105,6 +105,8 @@ class r_renderer_c: public r_IRenderer, public conCmdHandler_c {
 	conVar_c*	r_compress = nullptr;
 	conVar_c*	r_screenshotFormat = nullptr;
 	conVar_c*	r_layerDebug = nullptr;
+	conVar_c*   r_layerOptimize = nullptr;
+	conVar_c*   r_layerShuffle = nullptr;
 
 	r_shaderHnd_c* whiteImage = nullptr;	// White image