Avoid material rebind when using skeleton

[clayjohn]

Fixes #37660 For 3.2. Because of the nullptr change, I need a different PR for each.

Changes to use the same logic as GLES3

godot/drivers/gles3/rasterizer_scene_gles3.cpp

Lines 2190 to 2200 in df87601

	if (prev_skeleton != skeleton) {
	if ((prev_skeleton == NULL) != (skeleton == NULL)) {
	state.scene_shader.set_conditional(SceneShaderGLES3::USE_SKELETON, skeleton != NULL);
	rebind = true;
	}
	if (skeleton) {
	glActiveTexture(GL_TEXTURE0 + storage->config.max_texture_image_units - 1);
	glBindTexture(GL_TEXTURE_2D, skeleton->texture);
	}
	}

Previously, every time a valid skeleton was used, a rebind would occur. Now, a rebind on occurs when the previous skeleton was null and the current is valid or vice versa (i.e. state changes only when state needs to change)

I am not sure why (prev_skeleton != skeleton) evaluates to true every frame, but I assume it has to do with how GLES2 treats skeletons, it seems to make a slightly different skeleton for each mesh instead of sharing one across many.

[akien-mga]

Thanks!

[akien-mga]

For 3.2. Because of the nullptr change, I need a different PR for each.

Note that nullptr is still supported on 3.2 (which builds with -std=gnu++14), so it would have been OK to cherry-pick the master commit too.