Allow installation of extra platform plugins

Resolves linuxdeploy#88

Author: asl

Diff for: README.md

@@ -57,6 +57,7 @@ Just like all linuxdeploy plugins, the Qt plugin's behavior can be configured so
 **Qt specific:**
 - `$QMAKE=/path/to/my/qmake`: use another `qmake` binary to detect paths of plugins and other resources (usually doesn't need to be set manually, most Qt environments ship scripts changing `$PATH`)
 - `$EXTRA_QT_PLUGINS=pluginA;pluginB`: Plugins to deploy even if not found automatically by linuxdeploy-plugin-qt
+- `$EXTRA_PLATFORM_PLUGINS=platformA;platformB`: Platforms to deploy in addition to `libqxcb.so`. Platform must be available from `QT_INSTALL_PLUGINS/platforms`.
 
 QML related:
 - `$QML_SOURCES_PATHS`: directory containing the application's QML files — useful/needed if QML files are "baked" into the binaries. `$QT_INSTALL_QML` is prepended to this list internally.

Diff for: ci/test.sh

@@ -87,4 +87,15 @@ pushd linuxdeploy-plugin-qt-examples/QtWidgetsApplication
         "$linuxdeploy_bin" --appdir "$PWD"/AppDir --plugin qt --output appimage || exit 1
         mv -v *AppImage "$build_dir" || exit 1
     popd
+
+    mkdir build-platforms
+    pushd build-platforms
+        export EXTRA_PLATFORM_PLUGINS="libqoffscreen.so;libqminimal.so"
+        qmake CONFIG+=release PREFIX=/usr ../QtWidgetsApplication.pro || exit 1
+        INSTALL_ROOT="$PWD"/AppDir make install || exit 1
+
+        "$LINUXDEPLOY_BIN" --appdir "$PWD"/AppDir --plugin qt --output appimage || exit 1
+        APPIMAGE=(*AppImage)
+        mv -v "${APPIMAGE[0]}" "$BUILD_DIR/offscreen-${APPIMAGE[0]}" || exit 1
+    popd
 popd

Diff for: src/deployers/PlatformPluginsDeployer.cpp

@@ -1,5 +1,6 @@
 // library headers
 #include <linuxdeploy/core/log.h>
+#include <linuxdeploy/util/util.h>
 #include <boost/filesystem.hpp>
 
 // local headers
@@ -17,9 +18,19 @@ bool PlatformPluginsDeployer::deploy() {
 
     ldLog() << "Deploying platform plugins" << std::endl;
 
+    // always deploy default platform
     if (!appDir.deployLibrary(qtPluginsPath / "platforms/libqxcb.so", appDir.path() / "usr/plugins/platforms/"))
         return false;
 
+    // deploy extra platform plugins, if any
+    if (const auto* const platformPluginsFromEnvData = getenv("EXTRA_PLATFORM_PLUGINS")) {
+        for (const auto& platformToDeploy : linuxdeploy::util::split(std::string(platformPluginsFromEnvData), ';')) {
+            ldLog() << "Deploying extra platform plugin: " << platformToDeploy << std::endl;
+            if (!appDir.deployLibrary(qtPluginsPath / "platforms" / platformToDeploy, appDir.path() / "usr/plugins/platforms/"))
+                return false;
+        }
+     }
+
     for (bf::directory_iterator i(qtPluginsPath / "platforminputcontexts"); i != bf::directory_iterator(); ++i) {
         if (!appDir.deployLibrary(*i, appDir.path() / "usr/plugins/platforminputcontexts/"))
             return false;