egl: Add bindings for `EGLImage`

[MarijnS95]

After adding a DRM example in #1691 I've on-and-off been using raw DRM as a base to allocate and present/flip GBM images (gbm_bo) by creating a gbm_surface (some sort of swapchain containing multiple gbm_bo's) that one can import into an EGLSurface (already supported in glutin), and once surface.swap_buffers() is called after rendering a new gbm_surface::lock_front_buffer() becomes available to flip to DRM (or to give to a compositor?) [^1].

[^1]: I'd like to submit these as extra glutin_examples at some point.

However, OpenGL via EGL also support rendering to individual gbm_bos, by importing them into an EGLImage via EGL_KHR_image_pixmap (iirc resummarized in EGL_KHR_image), and subsequently binding that to a new framebuffer via OES_EGL_image.

I've written some code to test this out successfully, and would like to ask before stuffing this into a PR: should we add an EGLImage representation to glutin's EGL backend (even though the code can already be written in "user code" against egl_display.raw_display() and egl_display.egl())? Furthermore, is there a similarity in other context APIs?

Note that eglCreateImage() can also be used to create EGLImages from/for various GL_TEXTURE_..., if I read it correctly.

[MarijnS95]

@kchibisov you removing the ? from this title counts as a "yes, it's okay to add an Image wrapper to the EGL backend" answer?

If so, I'll open a PR with what I've been writing thus far.

[kchibisov]

I don't see why not, if there's a EGL api that could be wrapped, then I don't see why shouldn't we.