refactor(render3d): render targets and passes behind gpu.ludic

Framebuffers and their attachments, renderbuffers (multisampled too), draw and read buffers,
completeness checks, blits, viewports, clears, the screen framebuffer, the multisample enable,
the wireframe switch and GL error checks now go through gpu_fb_* / gpu_rb_* / gpu_viewport /
gpu_clear / gpu_blit / gpu_check, and no other file names them. Each call is the one GL call it
replaces, in the same order: the fixed viewpoints render bit-identically and the game's
self-tests report exactly what they did.

What is attached to each framebuffer - colour slots, a depth texture or one layer of an array,
renderbuffers and their samples - is recorded as it is attached, for a backend that builds
render passes and image views. gpu_read_screen is the frame read-back a photograph takes.

R3D_GLCHECK=1 checks, around every draw, clear and blit, that the bound framebuffer is complete
and that no error is left behind, naming the framebuffer. It has already narrowed the old
"gl error 1286 at terrain shadow bake": the error is pending before a draw after the map self-
test, so it comes from a call that is not a draw, clear or blit.

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

packages/ludic.render3d/overlay.ludic

@@ -155,8 +155,8 @@ function ov_glyph(cp: int) -> int {
 # start drawing onto the screen: blending on, depth off
 function ov_begin() -> void {
   if not ov_ready { return }
-  gl_bind_framebuffer(GL_FRAMEBUFFER, gl_screen_fbo())
-  gl_viewport(0, 0, gl_w, gl_h)
+  gpu_fb_bind(gpu_screen_fb())
+  gpu_viewport(0, 0, gl_w, gl_h)
   gpu_depth_test(false)
   gpu_cull(false)
   gpu_blend(true)