fix: Exclude raft layers from slow-down layer count

[zackaree-shen]

Summary

Fixes a bug where slow-down layers (slow_down_layers) incorrectly counted raft layers, causing the model's actual first few layers to receive insufficient speed reduction.

Root Cause

In GCode::_extrude(), the slow-down layer lerp used raw layer_id() which includes raft layers in its counting. With 3 raft layers and slow_down_layers=5, the 3 raft layers consumed 3 slow-down slots, leaving only 2 for the actual model layers.

Fix

• Subtract raft_layers() from layer_id() so slow-down counting starts from the first model layer
• Change _layer > 0 to _layer >= 0 so the first model layer over raft also receives proper initial speed

Changes

File	Lines
src/libslic3r/GCode.cpp	+5, -2

Behavior (example: 3 raft layers + slow_down_layers=5)

Before (bug)	After (fix)
Raft layers 1-2: slow-down applied	Raft layers: normal speed
Model layers 1-2: slow-down (only 2 of 5)	Model layers 1-5: full slow-down gradient

Verification

• Compiled successfully with MSVC 19.44 (VS 2022)
• Application starts and runs correctly