Move single/double-buffer filtering to backends

There is no need to enumerate framebuffer configs that will just be
filtered out later by the GLFW_DOUBLEBUFFER hard constraint.

src/context.c

@@ -196,12 +196,6 @@ const _GLFWfbconfig* _glfwChooseFBConfig(const _GLFWfbconfig* desired,
             continue;
         }
 
-        if (desired->doublebuffer != current->doublebuffer)
-        {
-            // Double buffering is a hard constraint
-            continue;
-        }
-
         // Count number of missing buffers
         {
             missing = 0;