Backport fixes relevant to rendering into release

• Fix leading newlines in shader code !10954 (merged)
• Fixed various clang -Wmissing-variable-declarations warnings !10695 (merged)
• Remove globals that use vtkNew<>. !10532 (merged)
• OpenGL2: Use #ifdef instead of #if to fix compiler error !10664 (merged)
• Disable linear filtering for depth textures in GLES 3.0 emscripten !11135 (merged)
• Fix point size bug in charts with GLES 3.0 !11134 (merged)
• Make PrintSelf output useful member variables in Context2D classes !11133 (merged)
• Add null check in vtkContextScene::SetTransform !11133 (merged)
• Fix vtkContextView::SetScene with smart pointer macro !11138 (merged)
• vtkLabelHierarchy: re-compute hierarchy when points have changed !11137 (merged)
• Reduce color banding for gradient backgrounds !10607 (merged)
• Glyph3DMapper: Fix display attribute inheritance logic !10541 (merged)
• vtkOpenGLES30PolyDataMapper: Do not cast gl_VertexID to float !10503 (merged)
• gles: Avoid the noperspective keyword in shaders !10466 (merged)
• rendering: Do not offset vertices for drawing polygon edges !10403 (merged)
• ReadPixels: Check for errors only when VTK is configured to report errors !10646 (merged)
• OIT: Do not blit depth buffer when multisampling in GLES context !10637 (merged)
• vtkOpenGLState: Check for errors after glBlitFramebuffer !10637 (merged)