Trade: SceneFieldData::size() should be a std::size_t, not UnsignedLong.

Not sure what was I thinking here -- if a field size wouldn't fit into a
32bit number, it won't fit into memory of a 32bit system anyway, so
there's no real use for the size to be returned as 64bit always.
Internally it *is* stored as a 64bit number, yes, to have compatible
binary layout on 32bit and 64bit systems, but that doesn't mean the
public API should return that too. And SceneData::fieldSize() is
std::size_t, so this feels really like an accidental brainfart.

The changed return type also means a lot of existing code doesn't need
to do any explicit casting to std::size_t anymore. Yay.

src/Magnum/SceneTools/Implementation/combine.h

@@ -159,7 +159,7 @@ inline Trade::SceneData combineFields(const Trade::SceneMappingType mappingType,
         sharedTrsMapping = itemViewOffset;
         arrayAppend(items, InPlaceInit,
             NoInit,
-            std::size_t(fields[sharedSceneFieldIds.trs[0]].size()),
+            fields[sharedSceneFieldIds.trs[0]].size(),
             mappingTypeSize,
             mappingTypeAlignment,
             itemViews[itemViewOffset].types);
@@ -170,7 +170,7 @@ inline Trade::SceneData combineFields(const Trade::SceneMappingType mappingType,
         sharedMeshMaterialMapping = itemViewOffset;
         arrayAppend(items, InPlaceInit,
             NoInit,
-            std::size_t(fields[sharedSceneFieldIds.meshMaterial[0]].size()),
+            fields[sharedSceneFieldIds.meshMaterial[0]].size(),
             mappingTypeSize,
             mappingTypeAlignment,
             itemViews[itemViewOffset].types);
@@ -220,7 +220,7 @@ inline Trade::SceneData combineFields(const Trade::SceneMappingType mappingType,
             itemViewMappings[i].first() = itemViewOffset;
             arrayAppend(items, InPlaceInit,
                 NoInit,
-                std::size_t(field.size()),
+                field.size(),
                 mappingTypeSize,
                 mappingTypeAlignment,
                 itemViews[itemViewOffset].types);
@@ -236,13 +236,13 @@ inline Trade::SceneData combineFields(const Trade::SceneMappingType mappingType,
         if(fieldType == Trade::SceneFieldType::Bit) {
             arrayAppend(items, InPlaceInit,
                 NoInit,
-                Containers::Size2D{std::size_t(field.size()), field.fieldArraySize() ? field.fieldArraySize() : 1},
+                Containers::Size2D{field.size(), field.fieldArraySize() ? field.fieldArraySize() : 1},
                 itemViews[itemViewOffset].bits);
             ++itemViewOffset;
         } else {
             arrayAppend(items, InPlaceInit,
                 NoInit,
-                std::size_t(field.size()), sceneFieldTypeSize(fieldType)*(field.fieldArraySize() ? field.fieldArraySize() : 1),
+                field.size(), sceneFieldTypeSize(fieldType)*(field.fieldArraySize() ? field.fieldArraySize() : 1),
                 sceneFieldTypeAlignment(fieldType),
                 itemViews[itemViewOffset].types);
             ++itemViewOffset;

src/Magnum/SceneTools/Implementation/convertToSingleFunctionObjects.h

@@ -124,11 +124,11 @@ inline Trade::SceneData convertToSingleFunctionObjects(const Trade::SceneData& s
             /** @todo wow this placeholder construction is HIDEOUS */
             fields[i] = Trade::SceneFieldData{field.name(),
                 field.mappingType(),
-                Containers::ArrayView<const UnsignedInt>{nullptr, std::size_t(field.size() + fieldsToCopyAdditionCount[*fieldToCopy])},
+                Containers::ArrayView<const UnsignedInt>{nullptr, field.size() + fieldsToCopyAdditionCount[*fieldToCopy]},
                 field.fieldType(),
                 Containers::StridedArrayView1D<const void>{
                     {nullptr, ~std::size_t{}},
-                    std::size_t(field.size() + fieldsToCopyAdditionCount[*fieldToCopy]),
+                    field.size() + fieldsToCopyAdditionCount[*fieldToCopy],
                     std::ptrdiff_t((field.fieldArraySize() ? field.fieldArraySize() : 1)*sceneFieldTypeSize(field.fieldType()))
                 },
                 field.fieldArraySize(),
@@ -140,7 +140,7 @@ inline Trade::SceneData convertToSingleFunctionObjects(const Trade::SceneData& s
         } else if(field.name() == Trade::SceneField::Parent) {
             /** @todo some nicer constructor for placeholders once this is in
                 public interest */
-            fields[i] = Trade::SceneFieldData{Trade::SceneField::Parent, Containers::ArrayView<const UnsignedInt>{nullptr, std::size_t(field.size() + objectsToAdd)}, Containers::ArrayView<const Int>{nullptr, std::size_t(field.size() + objectsToAdd)},
+            fields[i] = Trade::SceneFieldData{Trade::SceneField::Parent, Containers::ArrayView<const UnsignedInt>{nullptr, field.size() + objectsToAdd}, Containers::ArrayView<const Int>{nullptr, field.size() + objectsToAdd},
                 /* If the field is ordered, we preserve that. But if it's
                    implicit, we can't. */
                 field.flags() & ~(Trade::SceneFieldFlag::ImplicitMapping & ~Trade::SceneFieldFlag::OrderedMapping)

src/Magnum/Trade/SceneData.h

@@ -1324,7 +1324,7 @@ class MAGNUM_TRADE_EXPORT SceneFieldData {
         constexpr SceneField name() const { return _name; }
 
         /** @brief Number of entries */
-        constexpr UnsignedLong size() const { return _size; }
+        constexpr std::size_t size() const { return _size; }
 
         /** @brief Object mapping type */
         constexpr SceneMappingType mappingType() const {