Hardened checks for OpenGL ES version.

There is no ES 2.5 or 3.01 and this way we can properly check for
garbage returned by `glGet(GL_VERSION_M*OR)` (e.g. version 2674.-2O8539
would pass here).

src/Magnum/Context.cpp

@@ -346,9 +346,9 @@ Context::Context() {
     #ifndef MAGNUM_TARGET_GLES
     if(!isVersionSupported(Version::GL210))
     #elif defined(MAGNUM_TARGET_GLES2)
-    if(!isVersionSupported(Version::GLES200))
+    if(_version != Version::GLES200)
     #else
-    if(!isVersionSupported(Version::GLES300))
+    if(_version != Version::GLES300)
     #endif
     {
         #ifndef MAGNUM_TARGET_GLES