#366 Value truncation warnings in Windows x64

[Anonymous]

When building QWT on Windows x64, we get a lot of data truncation warnings due to the fact, that 'size_t' on this platform is larger than 'int' (64 vs. 32 bit). The attached patch avoids this by adding explicit casts.

Note that it also changes the signature of 'QwtSyntheticPointData::x()', which introduces an ABI mismatch. So check, if that is an issue...