Check if an extension is supported, before calling function pointer returned by eglGelProcAddress

[jschwe]

A non-NULL return value does not guarantee that an extension function is actually supported at runtime. The client must also make a corresponding query, such as glGetString(GL_EXTENSIONS) for OpenGL and OpenGL ES extensions; vgGetString(VG_EXTENSIONS) for OpenVG extensions; eglQueryString(display, EGL_EXTENSIONS); or query the EGL or client API version for non-extension functions, to determine if a function is supported by EGL or a specific client API context.

Source: https://registry.khronos.org/EGL/sdk/docs/man/html/eglGetProcAddress.xhtml

I haven't ran into any specific problem, but currently we are not doing this check.

[sagudev]

If we move to glow (from gl_generator), it will do this checks automatically (if functions are not loaded it will panic), so it will be safer. See servo/servo#33539