The High DPI support is enabled by default, so that the renderer use the
full definition of High DPI screens.
However, there are still mouse coordinates problems on some MacOS having
High DPI support (but not all), so expose a way to disable it.
The server is currently a JAR, but it may ba an APK or a DEX, so the
variable name should not contain the type.
Rename the environment variable, the Meson options and the C
definitions.
Expose a 'prebuilt_server' option to pass the path of the prebuilt
binary, so that the build does not require Android SDK.
Usage:
meson builddir -Dprebuilt_server=/tmp/my_prebuilt_server.jar
Since Meson 0.44, subproject_dir may not be '.' anymore. This implies we
must move app/ and server/ to a subprojects/ directory, which requires
to also change some gradle files.
Instead, just use subdir(), with options to disable building of the app
or the server.
