render3d: the deterministic work as bytes a bake keeps, and the textures made from them

Each pair runs the work as the renderer always has and reads the result back, or makes the same
textures and fills them from bytes instead: impostor_bytes / impostor_from_bytes (and impostor_fill,
for a fog that opens past a layer's cards), terrain_shadow_bytes / terrain_shadow_from_bytes,
sky_ibl_bytes / sky_ibl_from_bytes (sky_precompute split into sky_ibl_make and the convolutions).
A bundle is a word count, a word length per part and the parts (bake_pack / bake_unpack), checked
against the textures' shapes before any byte is used. gpu_vk_readback.ludic reads a texture's level 0,
every layer, as stored; r3d_baked_read reads a bake's file (ludic.base's LBAK header: key and version
must match) since render3d cannot import ludic.base. Nothing calls them yet: behaviour unchanged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.render3d/gpu_vk_readback.ludic

@@ -0,0 +1,61 @@
+# gpu_vk_readback.ludic — a texture's level 0, every layer, read back as the bytes it stores, and put back
+# from them: what a build-time bake keeps of a texture the renderer made (bake_parts.ludic).
+
+# the bytes a texture's level 0 takes, every layer, in the format it is stored in
+function gpu_tex_byte_size(render3d_st: Render3dState, tex: int) -> int {
+  let ifmt = render3d_st.gvk_tex_glfmt[tex]
+  return render3d_st.gvk_tex_dims_w[tex] * render3d_st.gvk_tex_dims_h[tex] * render3d_st.gvk_tex_layers[tex] * gvk_channels(ifmt) * gvk_channel_bytes(ifmt)
+}
+
+# level 0 of layer `layer` into `out` (w * h * channels * bytes), as stored
+function gvk_tex_read_layer(render3d_st: mut Render3dState, tex: int, layer: int, out: pointer) -> bool {
+  let ifmt = render3d_st.gvk_tex_glfmt[tex]
+  let w = render3d_st.gvk_tex_dims_w[tex]
+  let h = render3d_st.gvk_tex_dims_h[tex]
+  let layers = render3d_st.gvk_tex_layers[tex]
+  let n = w * h * gvk_channels(ifmt) * gvk_channel_bytes(ifmt)
+  let src = gvk_staging(render3d_st, n, VK_BUFFER_USAGE_TRANSFER_DST_BIT)
+  if src == null { return false }
+  let image = render3d_st.gvk_tex_image[tex]
+  let levels = render3d_st.gvk_tex_levels[tex]
+  let cb = gvk_once_begin(render3d_st)
+  gvk_barrier(render3d_st, cb, image, false, 0, levels, layers, VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL, VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL)
+  let bic = gvk_tmp(render3d_st, VkBufferImageCopy_sizeof)
+  Vk.zero(bic, VkBufferImageCopy_sizeof)
+  Vk.put_i32(bic, VkBufferImageCopy_imageSubresource + VkImageSubresourceLayers_aspectMask, VK_IMAGE_ASPECT_COLOR_BIT)
+  Vk.put_i32(bic, VkBufferImageCopy_imageSubresource + VkImageSubresourceLayers_baseArrayLayer, layer)
+  Vk.put_i32(bic, VkBufferImageCopy_imageSubresource + VkImageSubresourceLayers_layerCount, 1)
+  Vk.put_i32(bic, VkBufferImageCopy_imageExtent + VkExtent3D_width, w)
+  Vk.put_i32(bic, VkBufferImageCopy_imageExtent + VkExtent3D_height, h)
+  Vk.put_i32(bic, VkBufferImageCopy_imageExtent + VkExtent3D_depth, 1)
+  Vk.cmd_copy_image_to_buffer(cb, image, VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL, render3d_st.gvk_st_buf, 1, bic)
+  gvk_barrier(render3d_st, cb, image, false, 0, levels, layers, VK_IMAGE_LAYOUT_TRANSFER_SRC_OPTIMAL, VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL)
+  let ok = gvk_once_end(render3d_st, cb)
+  if ok { mem_copy(out, src, n) }
+  gvk_staging_free(render3d_st)
+  return ok
+}
+
+# every layer's level 0 into `out` (gpu_tex_byte_size bytes), layer after layer
+function gpu_tex_read_all(render3d_st: mut Render3dState, tex: int, out: pointer) -> bool {
+  let layers = render3d_st.gvk_tex_layers[tex]
+  let per = gpu_tex_byte_size(render3d_st, tex) / max(layers, 1)
+  for l in 0 .. layers { if not gvk_tex_read_layer(render3d_st, tex, l, mem_off(out, l * per)) { return false } }
+  return true
+}
+
+# level 0 of every layer written from bytes read back by gpu_tex_read_all; the texture's storage must
+# already be what it was (same format, size and layers). Mips, where it has them, are the caller's
+function gpu_tex_write_all(render3d_st: mut Render3dState, tex: int, data: pointer) -> bool {
+  let ifmt = render3d_st.gvk_tex_glfmt[tex]
+  let c = gvk_channels(ifmt)
+  let b = gvk_channel_bytes(ifmt)
+  var fmt = GL_RGBA
+  if c == 1 { fmt = GL_RED }
+  if c == 2 { fmt = GL_RG }
+  if c == 3 { fmt = GL_RGB }
+  var ty = GL_UNSIGNED_BYTE
+  if b == 2 { ty = GL_HALF_FLOAT }
+  if b == 4 { ty = GL_FLOAT }
+  return gvk_tex_upload(render3d_st, tex, ifmt, render3d_st.gvk_tex_dims_w[tex], render3d_st.gvk_tex_dims_h[tex], render3d_st.gvk_tex_layers[tex], fmt, ty, data)
+}