overlay: implement dx11 backend (#707)

## Link to GitHub Issue or related Pull Request, if one exists
Fixes #134 

## Description of change
Add ImGui DX11 backend implementation, and integrate into the spice
overlay.

The tricky part is that some of the Unity games:

1. Launch multiple windows, and
2. Can be resized (even full screen games launch at a certain resolution
and then expand to fit desktop resolution)
3. lazy load DX11 DLLs

This PR also adds screenshot functionality, but screen resize is not
implemented.

## Testing

Seems to be working for most games. Need to do final tests for:

- [x] CCJ (attract movie plays as well)
- [x] Busou Shinki (title movie plays)
- [x] MFG
- [x] QuizKnock
- [x] Polaris Chord
- [x] check dx9 for regression

src/spice2x/build_all.sh

@@ -172,7 +172,7 @@ time (
 		fi
 		mkdir -p ${BUILDDIR_WINXP_32}
 		pushd ${BUILDDIR_WINXP_32} > /dev/null
-		CXXFLAGS="$CXXFLAGS -DSPICE_XP=1" cmake -G "Ninja" -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN_WINXP_32} -DCMAKE_BUILD_TYPE=${BUILD_TYPE} "$OLDPWD" && ninja ${TARGETS_XP32}
+		cmake -G "Ninja" -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN_WINXP_32} -DCMAKE_BUILD_TYPE=${BUILD_TYPE} -DSPICE_XP=ON "$OLDPWD" && ninja ${TARGETS_XP32}
 		popd > /dev/null
 
 		# 64 bit Windows XP
@@ -185,7 +185,7 @@ time (
 		fi
 		mkdir -p ${BUILDDIR_WINXP_64}
 		pushd ${BUILDDIR_WINXP_64} > /dev/null
-		CXXFLAGS="$CXXFLAGS -DSPICE_XP=1" cmake -G "Ninja" -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN_WINXP_64} -DCMAKE_BUILD_TYPE=${BUILD_TYPE} "$OLDPWD" && ninja ${TARGETS_XP64}
+		cmake -G "Ninja" -DCMAKE_TOOLCHAIN_FILE=${TOOLCHAIN_WINXP_64} -DCMAKE_BUILD_TYPE=${BUILD_TYPE} -DSPICE_XP=ON "$OLDPWD" && ninja ${TARGETS_XP64}
 		popd > /dev/null
 	else
 		echo ""