Make all vars initialized in constructors rather than at some later point.

Reported by PVS-Studio, many thanks to @alexesDev for collecting the
report.

src/Magnum/DebugTools/CompareImage.cpp

@@ -412,7 +412,7 @@ namespace Corrade { namespace TestSuite {
 
 using namespace Magnum;
 
-Comparator<DebugTools::CompareImage>::Comparator(Float maxThreshold, Float meanThreshold): _maxThreshold{maxThreshold}, _meanThreshold{meanThreshold} {
+Comparator<DebugTools::CompareImage>::Comparator(Float maxThreshold, Float meanThreshold): _maxThreshold{maxThreshold}, _meanThreshold{meanThreshold}, _max{}, _mean{} {
     CORRADE_ASSERT(meanThreshold <= maxThreshold,
         "DebugTools::CompareImage: maxThreshold can't be smaller than meanThreshold", );
 }

src/Magnum/GL/Context.h

@@ -656,7 +656,7 @@ class MAGNUM_GL_EXPORT Context {
         MAGNUM_GL_LOCAL bool isCoreProfileImplementationNV();
         #endif
 
-        void(*_functionLoader)();
+        void(*_functionLoader)(){};
         Version _version;
         #ifndef MAGNUM_TARGET_WEBGL
         Flags _flags;

src/Magnum/Platform/AbstractXApplication.h

@@ -266,9 +266,9 @@ class AbstractXApplication {
         typedef Containers::EnumSet<Flag> Flags;
         CORRADE_ENUMSET_FRIEND_OPERATORS(Flags)
 
-        Display* _display;
-        Window _window;
-        Atom _deleteWindow;
+        Display* _display{};
+        Window _window{};
+        Atom _deleteWindow{};
 
         std::unique_ptr<Implementation::AbstractContextHandler<GLConfiguration, Display*, VisualID, Window>> _contextHandler;
         std::unique_ptr<Platform::GLContext> _context;

src/Magnum/Text/AbstractFont.cpp

@@ -56,9 +56,9 @@ std::vector<std::string> AbstractFont::pluginSearchPaths() {
 }
 #endif
 
-AbstractFont::AbstractFont(): _size(0.0f) {}
+AbstractFont::AbstractFont() = default;
 
-AbstractFont::AbstractFont(PluginManager::AbstractManager& manager, const std::string& plugin): AbstractPlugin{manager, plugin}, _size{0.0f}, _lineHeight{0.0f} {}
+AbstractFont::AbstractFont(PluginManager::AbstractManager& manager, const std::string& plugin): AbstractPlugin{manager, plugin} {}
 
 bool AbstractFont::openData(const std::vector<std::pair<std::string, Containers::ArrayView<const char>>>& data, const Float size) {
     CORRADE_ASSERT(features() & Feature::OpenData,

src/Magnum/Text/AbstractFont.h

@@ -361,7 +361,7 @@ class MAGNUM_TEXT_EXPORT AbstractFont: public PluginManager::AbstractPlugin {
     #ifdef DOXYGEN_GENERATING_OUTPUT
     private:
     #endif
-        Float _size, _ascent, _descent, _lineHeight;
+        Float _size{}, _ascent{}, _descent{}, _lineHeight{};
 };
 
 CORRADE_ENUMSET_OPERATORS(AbstractFont::Features)