demo: Add a unit test for provoking vertex generation

This test is small and simple so the algorithm ends up not cloning any
more vertices than it strictly needs to, which can happen on more
complex meshes.

demo/tests.cpp

@@ -1476,6 +1476,42 @@ static void tessellation()
 	assert(memcmp(tessib, expected, sizeof(expected)) == 0);
 }
 
+static void provoking()
+{
+	// 0 1 2
+	// 3 4 5
+	const unsigned int ib[] = {
+	    0, 1, 3,
+	    3, 1, 4,
+	    1, 2, 4,
+	    4, 2, 5,
+	    0, 2, 4,
+	    // clang-format :-/
+	};
+
+	unsigned int pib[15];
+	unsigned int pre[6 + 5]; // limit is vertex count + triangle count
+	size_t res = meshopt_generateProvokingIndexBuffer(pib, pre, ib, 15, 6);
+
+	unsigned int expectedib[] = {
+	    0, 5, 1,
+	    1, 4, 0,
+	    2, 4, 1,
+	    3, 4, 2,
+	    4, 5, 2,
+	    // clang-format :-/
+	};
+
+	unsigned int expectedre[] = {
+	    3, 1, 2, 5, 4, 0,
+	    // clang-format :-/
+	};
+
+	assert(res == 6);
+	assert(memcmp(pib, expectedib, sizeof(expectedib)) == 0);
+	assert(memcmp(pre, expectedre, sizeof(expectedre)) == 0);
+}
+
 static void quantizeFloat()
 {
 	volatile float zero = 0.f; // avoids div-by-zero warnings
@@ -1640,6 +1676,7 @@ void runTests()
 
 	adjacency();
 	tessellation();
+	provoking();
 
 	quantizeFloat();
 	quantizeHalf();