HiDPI splash screen support on Windows

Implement support of HiDPI splash screen loading on Windows.


The following methods from splashscreen_impl.h need to be implemented (see JDK-8043869):
void SplashSetScaleFactor(float);
char* SplashGetScaledImageName(const char*, const char*, float*);

There are some steps which should be considered:
- read scale factors: GetDpiForMonitor on Windows 8.1 and GetDesktopDpi on Windows 7

- It should be decided, which name conventions should be used for high-resolution splash images.
May be something like this: https://msdn.microsoft.com/en-us/library/windows/apps/xaml/hh965325.aspx
  name.scale-100.ext, name.scale-140.ext, name.scale-180.ext