Fix incorrect rendering of overlapping object in renderToArray() (#1306)

This commit adds a depth buffer in renderToArray().

This fixes the issue that overlapping objects are rendered incorrectly
when using renderToArray() on a GLViewWidget.

This might be related to issue #743.

pyqtgraph/opengl/GLViewWidget.py

@@ -498,6 +498,7 @@ class GLViewWidget(QtOpenGL.QGLWidget):
         self.makeCurrent()
         tex = None
         fb = None
+        depth_buf = None
         try:
             output = np.empty((w, h, 4), dtype=np.ubyte)
             fb = glfbo.glGenFramebuffers(1)
@@ -516,6 +517,12 @@ class GLViewWidget(QtOpenGL.QGLWidget):
             ## create teture
             glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, texwidth, texwidth, 0, GL_RGBA, GL_UNSIGNED_BYTE, data.transpose((1,0,2)))
 
+            # Create depth buffer
+            depth_buf = glGenRenderbuffers(1)
+            glBindRenderbuffer(GL_RENDERBUFFER, depth_buf)
+            glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT, texwidth, texwidth)
+            glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, depth_buf)
+
             self.opts['viewport'] = (0, 0, w, h)  # viewport is the complete image; this ensures that paintGL(region=...)
                                                   # is interpreted correctly.
             p2 = 2 * padding
@@ -545,6 +552,7 @@ class GLViewWidget(QtOpenGL.QGLWidget):
                 glDeleteTextures([tex])
             if fb is not None:
                 glfbo.glDeleteFramebuffers([fb])
+            if depth_buf is not None:
+                glDeleteRenderbuffers(1, [depth_buf])
 
         return output
-