I've bumped the version because SampleInstance is now called Channel, and due to minor changes to the C++ interface.

• All: Brings draw_rect in line with other draw_ methods (available in C++, available on Gosu::Window in Ruby)
• All: SampleInstance is now called Channel, but the old name still works in Ruby
• All: Update the stb_image dependency to fix crashes when loading 1-bit PNG files
• Windows: Update bundled SDL to 2.0.7
• macOS: Work around a compiler bug in Xcode 9+
• C++: Image and Sample have a default constructor now that creates an "empty" object; minor interface changes