rhi: gl: Fix not resetting color write bits when switching pipeline

This was visible in the triquadcube test when enabling transparent window
background: the cube on the left was rendered incorrectly because alpha
was not written out due to not setting glColorMask() back to the defaults
when switching to another pipeline.

Change-Id: I7a8c94072e0b68a58fffcc8c1a200e3940dcad44
Reviewed-by: Christian Strømme <christian.stromme@qt.io>

src/gui/rhi/qrhigles2.cpp

@@ -2258,6 +2258,7 @@ void QRhiGles2::executeBindGraphicsPipeline(QRhiGraphicsPipeline *ps)
         }
     } else {
         f->glDisable(GL_BLEND);
+        f->glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
     }
     if (psD->m_depthTest)
         f->glEnable(GL_DEPTH_TEST);