GPU Support

[pragmatrix]

This PR has the following objective:

• review and modernize the Vulkan feature APIs.
• support OpenGL
• support most of the GPU related SkSurface functions.
• extend GrContext with OpenGL support functions.
• render the skia-org example with OpenGL

Closes #50

[pragmatrix]

Sad, the examples can not be rendered with OpenGL when there is no possibility to open a (dummy) window. Locally it works under Windows (Radeon R7 200) MacOSX (Intel HD 3000) just fine, although the 5th example of the Path examples does not render a zero sized square cap on both machines, but this should be investigated separately.

Under Linux with XMing 6.9 on the Windows side (7 is available, but behind a donor gate, and the changelog does not tell if XInput2 is supported):

thread 'main' panicked at 'X server has XInput extension 1.4 but does not support XInput2'

So I assume this would work under Linux with a XInput2 capable X server, too.

I'll try to get Vulkan rendering to work in separate PR, and remove the OpenGL artifact generation from the CI as long we can't find a way to render OpenGL without a Window.

[Brooooooklyn]

In my experience, render OpenGL without a Window was simple under the macOS, but macOS does not support OpenGL anymore since OpenGL 4.1.
Under linux, it seems impossible to create OpenGL context without any window. I see most other applications choose Mesa to mock OpenGL API offscreen.
you can see: https://github.com/servo/rust-offscreen-rendering-context

[pragmatrix]

@Brooooooklyn Thanks for that, the offscreen context works better on Windows where the mouse pointer does not reappear, but it attempts to open an X Window under Linux.

I am preparing a PR to see how it goes works on MacOSX on Azure. Even if we can't use Azure for rendering OpenGL, it seems to be a better choice already.