GL: make it possible to pass a pre-filled Utility::Arguments to Context.

For example to support additional app-specific configuration.

src/Magnum/GL/Context.cpp

@@ -449,9 +449,11 @@ Context& Context::current() {
     return *currentContext;
 }
 
-Context::Context(NoCreateT, Int argc, const char** argv, void functionLoader()): _functionLoader{functionLoader}, _version{Version::None} {
+Context::Context(NoCreateT, Int argc, const char** argv, void functionLoader()): Context{NoCreate, Utility::Arguments{"magnum"}, argc, argv, functionLoader} {}
+
+Context::Context(NoCreateT, Utility::Arguments& args, Int argc, const char** argv, void functionLoader()): _functionLoader{functionLoader}, _version{Version::None} {
     /* Parse arguments */
-    Utility::Arguments args{"magnum"};
+    CORRADE_INTERNAL_ASSERT(args.prefix() == "magnum");
     args.addOption("disable-workarounds")
         .setHelp("disable-workarounds", "driver workarounds to disable\n      (see http://doc.magnum.graphics/magnum/opengl-workarounds.html for detailed info)", "LIST")
         .addOption("disable-extensions").setHelp("disable-extensions", "OpenGL extensions to disable", "LIST")

src/Magnum/GL/Context.h

@@ -638,6 +638,8 @@ class MAGNUM_GL_EXPORT Context {
         #endif
 
         explicit Context(NoCreateT, Int argc, const char** argv, void functionLoader());
+        explicit Context(NoCreateT, Utility::Arguments&& args, Int argc, const char** argv, void functionLoader()): Context{NoCreate, args, argc, argv, functionLoader} {}
+        explicit Context(NoCreateT, Utility::Arguments& args, Int argc, const char** argv, void functionLoader());
 
         bool tryCreate();
         void create();

src/Magnum/Platform/GLContext.h

@@ -108,6 +108,18 @@ class GLContext: public GL::Context {
         /** @overload */
         explicit GLContext(NoCreateT, Int argc, std::nullptr_t argv): GLContext{NoCreate, argc, static_cast<const char**>(argv)} {}
 
+        #ifndef DOXYGEN_GENERATING_OUTPUT
+        /* Used privately to inject additional command-line arguments */
+        explicit GLContext(NoCreateT, Utility::Arguments& args, Int argc, char** argv): GLContext{NoCreate, args, argc, const_cast<const char**>(argv)} {}
+        explicit GLContext(NoCreateT, Utility::Arguments& args, Int argc, std::nullptr_t argv): GLContext{NoCreate, args, argc, static_cast<const char**>(argv)} {}
+        explicit GLContext(NoCreateT, Utility::Arguments& args, Int argc, const char** argv):
+            #ifndef CORRADE_TARGET_EMSCRIPTEN
+            GL::Context{NoCreate, args, argc, argv, flextGLInit} {}
+            #else
+            GL::Context{NoCreate, args, argc, argv, nullptr} {}
+            #endif
+        #endif
+
         /**
          * @brief Construct the class without doing complete setup
          *