Fix version checks on OpenGL 2.1 and OpenGL ES 2.0.

Both don't have GL_{MAJOR,MINOR}_VERSION enums so we must decide whether
the version is older than OpenGL 3.0 / ES 3.0 either using
MAGNUM_TARGET_GLES2 preprocessor definition or checking for invalid enum
error and then we have to parse version string to ensure that the
version is not too old. On ES2 the version check wasn't present at all
(only optimistic hope), this makes it more hardened.

src/Magnum/Context.cpp

@@ -295,22 +295,60 @@ Context::Context() {
     }
     #endif
 
-    /* Version */
-    #ifndef MAGNUM_TARGET_GLES2
+    /* Get version */
+    #if defined(MAGNUM_TARGET_GLES) && !defined(MAGNUM_TARGET_GLES2)
     glGetIntegerv(GL_MAJOR_VERSION, &_majorVersion);
     glGetIntegerv(GL_MINOR_VERSION, &_minorVersion);
     #else
-    _majorVersion = 2;
-    _minorVersion = 0;
+
+    /* On GL 2.1 and ES 2.0 there is no GL_{MAJOR,MINOR}_VERSION, we have to
+       parse version string. On desktop GL we have no way to check version
+       without version (duh) so we work around that by checking for invalid
+       enum error. */
+    #ifndef MAGNUM_TARGET_GLES2
+    glGetIntegerv(GL_MAJOR_VERSION, &_majorVersion);
+    auto error = Renderer::error();
+    if(error == Renderer::Error::NoError)
+        glGetIntegerv(GL_MINOR_VERSION, &_minorVersion);
+    else
     #endif
+    {
+        #ifndef MAGNUM_TARGET_GLES2
+        CORRADE_ASSERT(error == Renderer::Error::InvalidEnum,
+            "Context: cannot retrieve OpenGL version:" << error, );
+        #endif
+
+        const std::string version = versionString();
+        #ifndef MAGNUM_TARGET_GLES
+        if(version.find("OpenGL 2.1") != std::string::npos)
+        #else
+        if(version.find("OpenGL ES 2.0") != std::string::npos)
+        #endif
+        {
+            _majorVersion = 2;
+            #ifndef MAGNUM_TARGET_GLES
+            _minorVersion = 1;
+            #else
+            _minorVersion = 0;
+            #endif
+        } else {
+            Error() << "Context: unsupported version string:" << version;
+            std::exit(1);
+        }
+    }
+    #endif
+
+    /* Compose the version enum */
     _version = Magnum::version(_majorVersion, _minorVersion);
 
+    /* Check that version retrieval went right */
     #ifndef CORRADE_NO_ASSERT
-    const auto error = Renderer::error();
+    error = Renderer::error();
     CORRADE_ASSERT(error == Renderer::Error::NoError,
         "Context: cannot retrieve OpenGL version:" << error, );
     #endif
 
+    /* Check that the version is supported (now it probably is, but be sure) */
     #ifndef MAGNUM_TARGET_GLES
     if(!isVersionSupported(Version::GL210))
     #elif defined(MAGNUM_TARGET_GLES2)