Draw, ...: use "auto" for LKSurface::SelectObject() return value

Prepare for using a different return type, one that is lighter.
Screen/LKSurface: return raw WIN32 handles from SelectObject()

Eliminates the overhead for creating temporary LKFont/LKBrush/LKPen
instances.  The executable size shrinks by 34 kB.
Merge pull request #10 from MaxKellermann/linux

Reduce LKSurface::SelectObject() overhead