Correct and revise OpenGL texture pooling (#2709)

test/util/lru_cache.test.cpp

@@ -0,0 +1,38 @@
+#include <mbgl/util/lru_cache.hpp>
+
+#include <gtest/gtest.h>
+
+#include <stdexcept>
+
+using namespace mbgl;
+
+TEST(LRU, LRU) {
+    LRU<int> lru;
+    EXPECT_TRUE(lru.empty());
+
+    lru.touch(1);
+    lru.touch(2);
+    lru.touch(3);
+    EXPECT_FALSE(lru.empty());
+    EXPECT_EQ(lru.size(), 3);
+
+    lru.remove(2);
+    lru.remove(77);
+    EXPECT_EQ(lru.evict(), 1);
+    EXPECT_FALSE(lru.empty());
+    EXPECT_EQ(lru.size(), 1);
+
+    lru.touch(4);
+    EXPECT_EQ(lru.evict(), 3);
+    EXPECT_FALSE(lru.empty());
+    EXPECT_EQ(lru.size(), 1);
+
+    EXPECT_FALSE(lru.isHit(3));
+    EXPECT_FALSE(lru.isHit(33));
+    EXPECT_TRUE(lru.isHit(4));
+
+    EXPECT_EQ(lru.evict(), 4);
+    EXPECT_TRUE(lru.empty());
+    EXPECT_FALSE(lru.isHit(4));
+    EXPECT_THROW(lru.evict(), std::runtime_error);
+}