Scratch resets and gated counters

- Terrain batch scratch marks reset in finally blocks so a failing kernel cannot grow the scratch
- Generator evaluation counters only count when generation counters are enabled
- Core tests run with generation counters enabled

core/build.gradle

@@ -178,6 +178,7 @@ dependencies {
 tasks.named('test').configure {
     maxHeapSize = '1g'
     jvmArgs('-XX:+ExitOnOutOfMemoryError')
+    systemProperty('iris.generation.counters', 'true')
 }
 
 java {