Add with_robustness and handle robustness on all implementations

src/headless.rs

@@ -5,6 +5,7 @@ use CreationError;
 use GlRequest;
 use GlContext;
 use PixelFormat;
+use Robustness;
 
 use gl_common;
 use libc;
@@ -43,6 +44,12 @@ impl HeadlessRendererBuilder {
         self
     }
 
+    /// Sets the robustness of the OpenGL context. See the docs of `Robustness`.
+    pub fn with_gl_robustness(mut self, robustness: Robustness) -> HeadlessRendererBuilder {
+        self.attribs.gl_robustness = robustness;
+        self
+    }
+
     /// Builds the headless context.
     ///
     /// Error should be very rare and only occur in case of permission denied, incompatible system,