ImFontAtlas: doesn't clear input data, left to the application. (#224)

Examples: DirectX9/11: hide os curosr if ImGui is drawing it (#155)

Examples: DirectX9/11: Fixed key mapping for down arrow.

Examples: Formatting #187

set pixel shader and vertex shader to NULL ...

Previous graphic render may use shaders,so set pixel shader and vertex shader to NULL to force use fixed pipeline.

Examples: DirectX9: fixed duplicate creation of vertex buffer. Size of static vertex buffer at top of the code.

Examples: Fixed Clang warnings in DirectX examples

Examples: MSVC: increase warning level from /W3 to /W4 for DirectX examples

Added io.KeyAlt + support in examples apps ...

Currently unused but supported by sample so that more people will have
it set up when menus needs them

Examples: reference implementations honors drawlist user callbacks.

Examples: DirectX9 example cleanup to match DirectX11 structure.

Examples: DirectX9 example application handle window resizing.

Examples: Refactored directx11_example to be easier to copy & paste into user application.

Examples: Tweaks.

Examples: Refactored directx9_example to be eaasier to copy & paste into user application.