GL: ensure to set depthmask to TRUE before rendering opaque shit

src/GPU3D_OpenGL.cpp

@@ -980,6 +980,8 @@ void RenderSceneChunk(int y, int h)
                         glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
                         glStencilMask(0xFF);
 
+                        glDepthMask(GL_TRUE);
+
                         RenderSinglePolygon(i);
                     }
 
@@ -1083,6 +1085,8 @@ void RenderSceneChunk(int y, int h)
                     glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
                     glStencilMask(0xFF);
 
+                    glDepthMask(GL_TRUE);
+
                     RenderSinglePolygon(i);
                 }