Platform: un-deprecate creation of Platform::Context without passing args.

I deprecated that because I wanted to be sure that I properly update all
my code to actually propagate the options. But now I see that this is
actually a valid case -- the engine being in a thread or in some other
way hidden from the outside *shouldn't* need access to argc/argv.

Also updated the docs to say it's completely harmless to propagate the
args.

src/Magnum/Context.h

@@ -97,24 +97,31 @@ also possible to create the context without using any `*Application` class
 using @ref Platform::Context subclass, see @ref platform documentation for more
 information.
 
-## Command-line options
+@section Context-command-line Command-line options
 
 The context is configurable through command-line options, that are passed
 either from the `Platform::*Application` classes or from the @ref Platform::Context
 class. Usage:
 
-    <application> [--magnum-help] [--magnum-disable-workarounds LIST] [--magnum-disable-extensions LIST] ...
+@code{.sh}
+<application> [--magnum-help] [--magnum-disable-workarounds LIST]
+              [--magnum-disable-extensions LIST] ...
+@endcode
 
 Arguments:
 
--   `...` -- main application arguments (see `-h` or `--help` for details)
--   `--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
+-   `...` --- main application arguments (see `-h` or `--help` for details)
+-   `--magnum-help` --- display this help message and exit
+-   `--magnum-disable-workarounds LIST` --- driver workarounds to disable (see
+    [src/Magnum/Implementation/driverSpecific.cpp](https://github.com/mosra/magnum/blob/master/src/Magnum/Implementation/driverSpecific.cpp)
+    for detailed info) (environment: `MAGNUM_DISABLE_WORKAROUNDS`)
+-   `--magnum-disable-extensions LIST` --- OpenGL extensions to disable
     (environment: `MAGNUM_DISABLE_EXTENSIONS`)
 
+Note that all options are prefixed with `--magnum-` to avoid conflicts with
+options passed to the application itself. Options that don't have this prefix
+are completely ignored, see documentation of the
+@ref Utility-Arguments-delegating "Utility::Arguments" class for details.
 */
 class MAGNUM_EXPORT Context {
     public:

src/Magnum/Platform/Context.h

@@ -50,9 +50,9 @@ class Context: public Magnum::Context {
          * is unsupported or any other error occurs, a message is printed to
          * output and the application exits. See @ref Context(NoCreateT, Int, char**)
          * and @ref tryCreate() for an alternative.
-         * @see @fn_gl{Get} with @def_gl{MAJOR_VERSION}, @def_gl{MINOR_VERSION},
-         *      @def_gl{CONTEXT_FLAGS}, @def_gl{NUM_EXTENSIONS},
-         *      @fn_gl{GetString} with @def_gl{EXTENSIONS}
+         * @see @ref Context-command-line, @fn_gl{Get} with @def_gl{MAJOR_VERSION},
+         *      @def_gl{MINOR_VERSION}, @def_gl{CONTEXT_FLAGS},
+         *      @def_gl{NUM_EXTENSIONS}, @fn_gl{GetString} with @def_gl{EXTENSIONS}
          */
         explicit Context(Int argc, const char** argv): Context{NoCreate, argc, argv} { create(); }
 
@@ -62,12 +62,16 @@ class Context: public Magnum::Context {
         /** @overload */
         explicit Context(Int argc, std::nullptr_t argv): Context{argc, static_cast<const char**>(argv)} {}
 
-        #ifdef MAGNUM_BUILD_DEPRECATED
-        /** @copybrief Context(Int, const char**)
-         * @deprecated Use @ref Context(Int, const char**) instead.
+        /**
+         * @brief Default constructor
+         *
+         * Equivalent to passing @cpp {0, nullptr} @ce to
+         * @ref Context(Int, const char**). Even if the command-line options
+         * are not propagated, it's still possible to affect the renderer
+         * behavior from the environment. See @ref Context-command-line for
+         * more information.
          */
-        CORRADE_DEPRECATED("use Context(Int, const char**) instead") explicit Context(): Context(0, nullptr) {}
-        #endif
+        explicit Context(): Context{0, nullptr} {}
 
         /**
          * @brief Construct the class without doing complete setup
@@ -90,6 +94,17 @@ class Context: public Magnum::Context {
         /** @overload */
         explicit Context(NoCreateT, Int argc, std::nullptr_t argv): Context{NoCreate, argc, static_cast<const char**>(argv)} {}
 
+        /**
+         * @brief Construct the class without doing complete setup
+         *
+         * Equivalent to passing @cpp {NoCreate, 0, nullptr} @ce to
+         * @ref Context(NoCreateT, Int, const char**). Even if the command-line
+         * options are not propagated, it's still possible to affect the
+         * renderer behavior from the environment. See @ref Context-command-line
+         * for more information.
+         */
+        explicit Context(NoCreateT): Context{NoCreate, 0, nullptr} {}
+
         /**
          * @brief Complete the context setup and exit on failure
          *