Implement texture-from-pixmap support via EGLImage

texture-from-pixmap is currently implemented by calling eglCreatePixmapSurface for the X11 pixmap and passing that surface to eglBindTexImage to bind it as a texture. This works fine if the EGL_NOKIA_texture_from_pixmap extension is present. However, there's two problems: First, this is a Nokia-specific extension. Second, creating the EGLSurface for the pixmap is expensive. It is much cheaper to use the EGL_KHR_image_pixmap extension instead, which is also a Khronos extension and thus more widely supported.