feat(render3d): r3d_present and r3d_screenshot - the end of a frame through the layer

A game ended its frame with Gl.swap() and shot it with Gl.screenshot, which tied it to OpenGL
and made it name Gl.* only so the parser would splice the GL runtime. gpu_present and
gpu_screenshot carry both (and name Gl.* themselves, so importing render3d is enough), and
r3d_present / r3d_screenshot are what a game calls. The Vulkan backend takes both over.

OpenGL frames byte-identical at the five viewpoints; 59 self-tests pass with no GL error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/ludic.render3d/gpu.ludic

@@ -220,6 +220,10 @@ function gpu_open(w: int, h: int, title: string) -> bool { return gl_open(w, h,
 function gpu_vsync(on: int) -> void { gl_vsync(on) }
 function gpu_renderer_name() -> string { return gl_get_string(GL_RENDERER) }
 function gpu_resize_check() -> bool { let r = gl_resize_check();  gpu_glcheck_after("the resize check");  return r }
+# the finished frame: presented to the window, or (headless) the GPU's work finished
+function gpu_present() -> void { Gl.swap() }
+# the frame as it will be presented, to a binary PPM with the top row first; before gpu_present
+function gpu_screenshot(path: string) -> bool { return Gl.screenshot(path: path) }
 
 var gpu_u_tmp: words = null
 function gpu_tmp() -> words { if gpu_u_tmp == null { gpu_u_tmp = words(4) };  return gpu_u_tmp }

packages/ludic.render3d/render.ludic

@@ -115,6 +115,11 @@ function r3d_draw_sky() -> void {
   gpu_depth_func(GL_LESS)
 }
 
+# The end of the game's frame, whichever backend draws it: a screenshot first if one is wanted,
+# then the present. A game calls these rather than Gl.swap / Gl.screenshot.
+function r3d_screenshot(path: string) -> bool { return gpu_screenshot(path) }
+function r3d_present() -> void { gpu_present() }
+
 # the drawable changed size: the camera's aspect and every screen-sized target follow
 function r3d_resize() -> void {
   cam_aspect = fr(gl_w, gl_h)