Cleaned up & fixed buffer alignment handling.

- Fixed ForceVectorAlignment (and possibly other call-sites) not
  setting minalign_.
- Fixed flipped alignment parameters in CopyTable (reflection).
- Made aligment for FlatBufferBuilder internal buffer configurable
  (useful when reading a constructed buffer directly).
- Ensured Alignment rounding is always up.

Change-Id: I33ca4887d92a09cb11a369c14a109f4b07ae707a
Tested: on Linux.

src/reflection.cpp

@@ -435,7 +435,7 @@ Offset<const Table *> CopyTable(FlatBufferBuilder &fbb,
             auto element_size = GetTypeSize(element_base_type);
             if (elemobjectdef && elemobjectdef->is_struct())
               element_size = elemobjectdef->bytesize();
-            fbb.StartVector(element_size, vec->size());
+            fbb.StartVector(vec->size(), element_size);
             fbb.PushBytes(vec->Data(), element_size * vec->size());
             offset = fbb.EndVector(vec->size());
             break;