[linux] Add GLFW-based proc resolving (#184)

Fixes #170

The default symbol resolver that is used when no specific resolver is given doesn't resolve according to OpenGL features available in the GLFW context. This causes errors for some users.

Author: GeertJohan

library/common/glfw/embedder.cc

@@ -205,6 +205,12 @@ static void GLFWClearCanvas(GLFWwindow *window) {
   glfwMakeContextCurrent(nullptr);
 }
 
+// Resolves the address of the specified OpenGL or OpenGL ES
+// core or extension function, if it is supported by the current context.
+static void *GLFWProcResolver(void *user_data, const char *name) {
+  return reinterpret_cast<void *>(glfwGetProcAddress(name));
+}
+
 // Spins up an instance of the Flutter Engine.
 //
 // This function launches the Flutter Engine in a background thread, supplying
@@ -228,6 +234,7 @@ static FlutterEngine RunFlutterEngine(
   config.open_gl.clear_current = GLFWClearContext;
   config.open_gl.present = GLFWPresent;
   config.open_gl.fbo_callback = GLFWGetActiveFbo;
+  config.open_gl.gl_proc_resolver = GLFWProcResolver;
   FlutterProjectArgs args = {};
   args.struct_size = sizeof(FlutterProjectArgs);
   args.assets_path = assets_path.c_str();