Point the layer compositor's GL context at the window's back buffer

Second half of the NVIDIA black screen (#3). With the visual fix the
compositor gets a window surface and drawables recycle, but the window
stays black: after CARenderer draws, the back buffer reads back black and
even a glClear to blue changes nothing. GL_DRAW_BUFFER and GL_READ_BUFFER
are GL_NONE.

CALayerContext -setSubwindow: creates the context and then notifies the
layer tree; CAMetalLayer -_setContext: makes the context current (to
create its textures) before any window surface is attached. Per
EGL_KHR_surfaceless_context, a context first made current without a
default framebuffer starts with DRAW_BUFFER = NONE, and attaching a
window later doesn't change it. NVIDIA follows that; Mesa resets it on
bind, which is why AMD works.

After CGLContextMakeCurrentAndAttachToWindow in -renderLayer:, set the
draw and read buffers to GL_BACK.

patches/cocotron.patch

@@ -8723,7 +8723,7 @@ index 5dcac115..5de4f03f 100644
 +
  @end
 diff --git a/QuartzCore/CALayerContext.m b/QuartzCore/CALayerContext.m
-index 5dce8c56..dee2b80c 100644
+index 5dce8c56..d76cdae4 100644
 --- a/QuartzCore/CALayerContext.m
 +++ b/QuartzCore/CALayerContext.m
 @@ -1,47 +1,58 @@
@@ -8879,7 +8879,7 @@ index 5dce8c56..dee2b80c 100644
  }
  
  - (void) assignTextureIdsToLayerTree: (CALayer *) layer {
-@@ -93,7 +148,22 @@
+@@ -93,8 +148,27 @@
          [self assignTextureIdsToLayerTree: child];
  }
  
@@ -8900,9 +8900,14 @@ index 5dce8c56..dee2b80c 100644
  - (void) renderLayer: (CALayer *) layer {
 +    CFTimeInterval renderLayerStart = CACurrentMediaTime();
      CGLContextMakeCurrentAndAttachToWindow(_glContext, _cglWindow);
++    // CAMetalLayer first makes this context current without a surface, which leaves GL_DRAW_BUFFER
++    // at GL_NONE on NVIDIA even once the window is attached (Mesa resets it).
++    glDrawBuffer(GL_BACK);
++    glReadBuffer(GL_BACK);
  
      glEnable(GL_DEPTH_TEST);
-@@ -118,27 +188,63 @@
+     glShadeModel(GL_SMOOTH);
+@@ -118,27 +192,63 @@
  
      [_deleteTextureIds removeAllObjects];
  
@@ -8971,7 +8976,7 @@ index 5dce8c56..dee2b80c 100644
  }
  
  - (void) startTimerIfNeeded {
-@@ -147,7 +253,7 @@
+@@ -147,7 +257,7 @@
                                                     target: self
                                                   selector: @selector(timer:)
                                                   userInfo: nil
@@ -8980,7 +8985,7 @@ index 5dce8c56..dee2b80c 100644
  }
  
  - (void) deleteTextureId: (NSNumber *) textureId {
-@@ -155,7 +261,10 @@
+@@ -155,7 +265,10 @@
  }
  
  - (void) flush {