add caching to GlStateManager#glViewport

common/src/main/java/net/caffeinemc/mods/sodium/client/data/config/MixinConfig.java

@@ -69,6 +69,8 @@ protected MixinConfig() {
 
         this.addMixinRule("features.render.particle", true);
 
+        this.addMixinRule("features.render.viewport", true);
+
         this.addMixinRule("features.render.world", true);
         this.addMixinRule("features.render.world.clouds", true);
         this.addMixinRule("features.render.world.sky", true);

common/src/main/java/net/caffeinemc/mods/sodium/mixin/features/render/viewport/GlStateManagerMixin.java

@@ -0,0 +1,30 @@
+package net.caffeinemc.mods.sodium.mixin.features.render.viewport;
+
+import com.mojang.blaze3d.opengl.GlStateManager;
+import org.lwjgl.opengl.GL11;
+import org.spongepowered.asm.mixin.Mixin;
+import org.spongepowered.asm.mixin.Overwrite;
+import org.spongepowered.asm.mixin.Unique;
+
+@Mixin(GlStateManager.class)
+public class GlStateManagerMixin {
+    @Unique private static int viewportX;
+    @Unique private static int viewportY;
+    @Unique private static int viewportWidth;
+    @Unique private static int viewportHeight;
+
+    /**
+     * @author Crosby
+     * @reason Viewport only changes a few times per frame
+     */
+    @Overwrite
+    public static void _viewport(int x, int y, int width, int height) {
+        if (x != viewportX || y != viewportY || width != viewportWidth || height != viewportHeight) {
+            viewportX = x;
+            viewportY = y;
+            viewportWidth = width;
+            viewportHeight = height;
+            GL11.glViewport(x, y, width, height);
+        }
+    }
+}

common/src/main/resources/sodium-common.mixins.json

@@ -72,6 +72,7 @@
     "features.render.model.ItemBlockRenderTypesMixin",
     "features.render.model.item.ItemRendererMixin",
     "features.render.particle.SingleQuadParticleMixin",
+    "features.render.viewport.GlStateManagerMixin",
     "features.render.world.sky.FogRendererMixin",
     "features.render.world.sky.ClientLevelMixin",
     "features.render.world.sky.LevelRendererMixin",