Allow usage of discard inside custom shader functions

[Chaosus]

Since #93358 got merged (which introduced call hierarchy checking), it's now easily possible to allow a user to use a discard keyword in custom user function in spatial/canvas_item shader types:

[djrain]

Is it possible to allow access to UV / SCREEN_UV and sdf functions as well? Just ran into this.

[Chaosus]

@djrain SDF functions (stage) are implemented in another PR : #93650, UV/SCREEN_UV still not.

[Calinou]

Tested locally (rebased on top of master 44fa552), it works as expected.

shader_type spatial;

void checkerboard(vec2 frag_coord) {
	if (int(frag_coord.x + frag_coord.y) % 2 == 0) {
		discard;
	}
}

void fragment() {
	checkerboard(FRAGCOORD.xy);
}

If you try to call the function in vertex(), it fails as expected:

Or if you try to call discard directly in vertex():