Ability to disable workarounds and extensions from environment.

Use MAGNUM_DISABLE_WORKAROUNDS and MAGNUM_DISABLE_EXTENSIONS environment
variables.

src/Magnum/Context.cpp

@@ -406,9 +406,11 @@ Context* Context::_current = nullptr;
 Context::Context(NoCreateT, Int argc, char** argv, void functionLoader()): _functionLoader{functionLoader}, _version{Version::None} {
     /* Parse arguments */
     Utility::Arguments args{"magnum"};
-    args.addOption("disable-workarounds").setHelpKey("disable-workarounds", "LIST")
-        .setHelp("disable-workarounds", "driver workarounds to disable\n      (see src/Magnum/Implementation/driverSpecific.cpp for detailed info)")
-        .addOption("disable-extensions").setHelpKey("disable-extensions", "LIST").setHelp("disable-extensions", "OpenGL extensions to disable")
+    args.addOption("disable-workarounds")
+        .setHelp("disable-workarounds", "driver workarounds to disable\n      (see src/Magnum/Implementation/driverSpecific.cpp for detailed info)", "LIST")
+        .addOption("disable-extensions").setHelp("disable-extensions", "OpenGL extensions to disable", "LIST")
+        .setFromEnvironment("disable-workarounds")
+        .setFromEnvironment("disable-extensions")
         .parse(argc, argv);
 
     /* Disable driver workarounds */

src/Magnum/Context.h

@@ -106,7 +106,9 @@ Arguments:
 -   `--magnum-help` -- display this help message and exit
 -   `--magnum-disable-workarounds LIST` -- driver workarounds to disable (see
     `src/Magnum/Implementation/driverSpecific.cpp` for detailed info)
+    (environment: `MAGNUM_DISABLE_WORKAROUNDS`)
 -   `--magnum-disable-extensions LIST` -- OpenGL extensions to disable
+    (environment: `MAGNUM_DISABLE_EXTENSIONS`)
 
 */
 class MAGNUM_EXPORT Context {