GLCanvas requires 8bit alpha channel for picking checksums.

src/slic3r/GUI/GLCanvas3DManager.cpp

@@ -290,7 +290,21 @@ GLCanvas3D* GLCanvas3DManager::get_canvas(wxGLCanvas* canvas)
 
 wxGLCanvas* GLCanvas3DManager::create_wxglcanvas(wxWindow *parent)
 {
-    int attribList[] = { WX_GL_RGBA, WX_GL_DOUBLEBUFFER, WX_GL_DEPTH_SIZE, 24, WX_GL_SAMPLE_BUFFERS, GL_TRUE, WX_GL_SAMPLES, 4, 0 };
+    int attribList[] = { 
+    	WX_GL_RGBA,
+    	WX_GL_DOUBLEBUFFER,
+    	// RGB channels each should be allocated with 8 bit depth. One should almost certainly get these bit depths by default.
+      	WX_GL_MIN_RED, 			8,
+      	WX_GL_MIN_GREEN, 		8,
+      	WX_GL_MIN_BLUE, 		8,
+      	// Requesting an 8 bit alpha channel. Interestingly, the NVIDIA drivers would most likely work with some alpha plane, but glReadPixels would not return
+      	// the alpha channel on NVIDIA if not requested when the GL context is created.
+      	WX_GL_MIN_ALPHA, 		0,
+    	WX_GL_DEPTH_SIZE, 		24,
+    	WX_GL_SAMPLE_BUFFERS, 	GL_TRUE,
+    	WX_GL_SAMPLES, 			4,
+    	0
+    };
 
     if (s_multisample == MS_Unknown)
     {