null2: null: rewrite and extend

- use GL to copy to display buffers, to work on any platform
- wait for vsync is optional
- refactor all buffer functionality into wnull_buffer.[ch]
- don't get confused if the current context is destroyed
- intercept glBindFramebuffer(GL_FRAMEBUFFER,0)
- intercept other erroneous framebuffer function calls
- a bunch of changes suggested by reviewer Emil Velikov

Signed-off-by: Frank Henigman <fjhenigman@google.com>

null1: waffle: add 'null' platform

The 'null' platform (WAFFLE_PLATFORM_NULL) works with EGL_PLATFORM_NULL,
KHR_surfaceless_context and EXT_image_dma_buf_import to hide surfaceless
operation behind a waffle window.
In this platform waffle windows contain gbm buffers which GLES renders
into via dmabuf -> EGL image -> GL framebuffer.
The gbm buffers can be displayed via DRM/KMS.

Signed-off-by: Frank Henigman <fjhenigman@google.com>