x11: Cast Window to EGLNativeWindowType using a C cast

reinterpret_cast<>() will fail if the types we cast from and to have
mismatching sizes.

Unfortunately, there are platforms that have Window and
EGLNativeWindowType of different size. This results in compilation
errors.

In order to work around those problematic platforms, this change
replaces reinterpret_cast cast with a c style cast.