Implement multiple-forms

[zwcloud]

old finished tweaks

• Move methods inside Form.GUI.cs to Application: NewFrame, EndFrame, Render and Log.
• Make all forms (including main form) update and render properly
  • Implement IWindow.Opacity
  • Update window movement implementation
  • issue: viewport resizing not invoked
  • Implement viewport resizing for rendering, especially for text rendering
• Inline Window.Render and re-organize MeshBuffer in Form level
  • Move MeshBuffer from Window to Form
  • Adjust MeshBuffer usage in Form: don't clear MeshBuffer every time foreach window when build it from Window.MeshList.
• Make IMGUI states GUIContext a global static object: It is a global variable at Application.ImGuiContext now.
• Singe OpenGL context for multiple native windows
• Win32OpengGLRenderer: decuple IRenderer from IWindow (make per native window state like hwnd, dc, viewport and scissor rect change when IRenderer::SetRenderingWindow)

New developement of multiple-forms will be based on the MultipleNativeWindowTest project.

An openGL renderer correctly renders Mesh on two win32 windows.

• Two regular window should render Mesh correctly. Fixed at 9802e0c.
• One main regular window A and one child window of A should render Mesh correctly. Fixed at 340b2be.

Misc

• Handle text framebuffers size corretly. Different form will have different viewport size. So we should use per-native-window off-screen framebuffer to render text. See InitializeTextRenderResources(size);. #73
• Update framebuffer when resizing a native window when rendering text #80 Text in popup window doesn't appear: text framebuffer size not updated
• Child window has no border rendered
• Non-main-form layout issue: inaccurate border/padding or content box location with 1 pixel offset #81
• Mouse/Keyboard input update logic should be reconsidered

This issue blocks #69