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Fixed MBC2 implementation (#36)

MBC2 only uses bit 8 to check between RAM enable and ROM bank, when writing to address < 0x4000. Both pandocs and sameboy agree on this.

[appveyor] Update VS version to 2017

It seems like VS 2015 may be generating bad code (see #20). Let's try
using 2017 instead.

Also, include SDL2.dll with the Windows bundles.

Add missing + in `add sp,%hhd`

Fix high DPI rendering on MacOS

According to the SDL2 docs, `SDL_GetWindowSize` returns the client
window size in screen coordinates, `SDL_GL_GetDrawableSize` returns the
size in pixels.

This change makes sure the `glViewport` calls always use pixels.

Update ImGui to v1.65