Print an error if OpenGL 4.1 isn't supported (cf #56) (#57)

* Print an error if OpenGL 4.1 isn't supported (cf #56)

* fix typo

README.md

@@ -16,8 +16,13 @@ The tool was originally conceived and implemented after the Revision 2014 demosc
 - F11 or Ctrl/Cmd-f: hide shader overlay
 - Alt-F4 or Shift+Escape: exbobolate your planet
 
+## Requirements
+On Windows, both DirectX 9 and 11 are supported.
+
+For the OpenGL version (for any platform), at least OpenGL 4.1 is required.
+
 ## Configuration
-Create a ```config.json``` with e.g. the following contents: (all fields are optional)
+Create a `config.json` with e.g. the following contents: (all fields are optional)
 ``` javascript
 {
   "window":{ /* default window size / state, if there's a setup dialog, it will override it */

src/platform_glfw/Renderer.cpp

@@ -188,6 +188,14 @@ namespace Renderer
   GLuint pbo[2];
 
   static void error_callback(int error, const char *description) {
+    switch (error) {
+    case GLFW_API_UNAVAILABLE:
+      printf("OpenGL is unavailable: ");
+      break;
+    case GLFW_VERSION_UNAVAILABLE:
+      printf("OpenGL 4.1 (the minimum requirement) is not available: ");
+      break;
+    }
     printf("%s\n", description);
   }
   void key_callback(GLFWwindow* window, int key, int scancode, int action, int mods);