MeshTools: don't allocate and memcpy zeros in interleave().

We have std::memset for that. What the hell I was thinking back then.

Found when trying to get rid of as many naked allocations and deletions
as possible. Also improved the test with some ASCII-art documentation.

src/MeshTools/Interleave.h

@@ -113,11 +113,9 @@ class Interleave {
 
         /* Fill gap with zeros */
         std::size_t writeOne(char* startingOffset, std::size_t gap) {
-            char* data = new char[gap]();
             for(std::size_t i = 0; i != _attributeCount; ++i)
-                memcpy(startingOffset+i*_stride, data, gap);
+                std::memset(startingOffset+i*_stride, 0, gap);
 
-            delete[] data;
             return gap;
         }
 

src/MeshTools/Test/InterleaveTest.cpp

@@ -122,13 +122,16 @@ void InterleaveTest::writeGaps() {
     CORRADE_COMPARE(attributeCount, std::size_t(3));
     CORRADE_COMPARE(stride, std::size_t(12));
     std::size_t size = attributeCount*stride;
+
     if(!Utility::Endianness::isBigEndian()) {
+        /*  byte, _____________gap, int___________________, short_____, _______gap */
         CORRADE_COMPARE(std::vector<char>(data, data+size), (std::vector<char>{
             0x00, 0x00, 0x00, 0x00, 0x03, 0x00, 0x00, 0x00, 0x06, 0x00, 0x00, 0x00,
             0x01, 0x00, 0x00, 0x00, 0x04, 0x00, 0x00, 0x00, 0x07, 0x00, 0x00, 0x00,
             0x02, 0x00, 0x00, 0x00, 0x05, 0x00, 0x00, 0x00, 0x08, 0x00, 0x00, 0x00
         }));
     } else {
+        /*  byte, _____________gap, ___________________int, _____short, _______gap */
         CORRADE_COMPARE(std::vector<char>(data, data+size), (std::vector<char>{
             0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 0x00, 0x06, 0x00, 0x00,
             0x01, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x04, 0x00, 0x07, 0x00, 0x00,