GL: don't use the slower splitOnWhitespace() when split() is enough.

The code was splitting on just a whitespace before the changes in
95b3f8578d (2021), so I suppose I just
accidentally copied the same code over from handling the
--disable-extensions and --disable-workarounds arguments, where the user
*is* allowed to use any whitespaces.

And given that just a space worked well for over a decade before,
there's no reason to pessimistically expect the extension strings
suddenly include tabs or newlines.

src/Magnum/GL/Context.cpp

@@ -1199,7 +1199,7 @@ Containers::Array<Containers::StringView> Context::extensionStrings() const {
     #if !defined(MAGNUM_TARGET_GLES) || defined(MAGNUM_TARGET_GLES2)
     {
         /* OpenGL 2.1 / OpenGL ES 2.0 doesn't have glGetStringi() */
-        return Containers::StringView{reinterpret_cast<const char*>(glGetString(GL_EXTENSIONS)), Containers::StringViewFlag::Global}.splitOnWhitespaceWithoutEmptyParts();
+        return Containers::StringView{reinterpret_cast<const char*>(glGetString(GL_EXTENSIONS)), Containers::StringViewFlag::Global}.splitWithoutEmptyParts(' ');
     }
     #endif
 }