Only check if fog is enabled once per frame

GL calls were being made every time `shouldRenderMesh` was called, oops.
This could cause worse performance with Nd than without it if `renderFog` is set
to `auto` (added in 0.1.5)

src/main/java/makamys/neodymium/renderer/NeoRenderer.java

@@ -61,6 +61,8 @@ public class NeoRenderer {
     public boolean forceRenderFog;
     public boolean hasIncompatibilities;
 
+    private boolean fogEnabled;
+
     private static int MAX_MESHES;
 
     private int VAO;
@@ -334,6 +336,8 @@ private void updateGLValues() {
         fogStartEnd.put(glGetFloat(GL_FOG_END));
 
         fogStartEnd.flip();
+
+        fogEnabled = GL11.glIsEnabled(GL11.GL_FOG) && !OFUtil.isFogOff();
     }
 
     private void updateUniforms(double alpha, int pass) {
@@ -623,14 +627,14 @@ private int getShaderProgram(int pass) {
         return ((forceRenderFog || isFogEnabled()) ? shaderProgramsFog : shaderProgramsNoFog)[pass];
     }
 
-    private static boolean isFogEnabled() {
+    private boolean isFogEnabled() {
         switch(Config.renderFog) {
         case TRUE:
             return true;
         case FALSE:
             return false;
         default:
-            return GL11.glIsEnabled(GL11.GL_FOG) && !OFUtil.isFogOff();
+            return fogEnabled;
         }
     }