Fixed serious issue with Context::is*Supported().

Some extensions are not part of any core OpenGL version (such as
EXT_direct_state_access, EXT_texture_filter_anisotropic etc.), thus they
have None for core version. Current mechanism in
Context::isExtensionSupported() is that it checks for core version first
and if that is supported, it doesn't check for the extension itself.

Until now Context::isVersionSupported(None) returned always true, which
meant that all non-core extensions were marked as supported
_everywhere_.

src/Context.h

@@ -36,7 +36,7 @@ namespace Implementation {
 
 /** @brief OpenGL version */
 enum class Version: GLint {
-    None = 0,                       /**< @brief Unspecified */
+    None = 0xFFFF,                  /**< @brief Unspecified */
     #ifndef MAGNUM_TARGET_GLES
     GL210 = 210,                    /**< @brief OpenGL 2.1 / GLSL 1.20 */
     GL300 = 300,                    /**< @brief OpenGL 3.0 / GLSL 1.30 */