Implement GPU backend detection for Blender compatibility

- Added functionality to detect GPU backends (HIP and NVIDIA) during runner registration, enhancing compatibility for Blender versions below 4.x.
- Introduced a new method, DetectAndStoreGPUBackends, to download the latest Blender and run a detection script, storing the results for future rendering decisions.
- Updated rendering logic to force CPU rendering when HIP is detected on systems with Blender < 4.x, ensuring stability and compatibility.
- Enhanced the Context structure to include flags for GPU detection status, improving error handling and rendering decisions based on GPU availability.

internal/runner/blender/binary.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"os"
 	"path/filepath"
+	"strings"
 
 	"jiggablend/internal/runner/api"
 	"jiggablend/internal/runner/workspace"
@@ -85,3 +86,42 @@ func (m *Manager) GetBinaryForJob(version string) (string, error) {
 	return m.GetBinaryPath(version)
 }
 
+// TarballEnv returns a copy of baseEnv with LD_LIBRARY_PATH set so that a
+// tarball Blender installation can find its bundled libs (e.g. lib/python3.x).
+// If blenderBinary is the system "blender" or has no path component, baseEnv is
+// returned unchanged.
+func TarballEnv(blenderBinary string, baseEnv []string) []string {
+	if blenderBinary == "" || blenderBinary == "blender" {
+		return baseEnv
+	}
+	if !strings.Contains(blenderBinary, string(os.PathSeparator)) {
+		return baseEnv
+	}
+	blenderDir := filepath.Dir(blenderBinary)
+	libDir := filepath.Join(blenderDir, "lib")
+	ldLib := libDir
+	for _, e := range baseEnv {
+		if strings.HasPrefix(e, "LD_LIBRARY_PATH=") {
+			existing := strings.TrimPrefix(e, "LD_LIBRARY_PATH=")
+			if existing != "" {
+				ldLib = libDir + ":" + existing
+			}
+			break
+		}
+	}
+	out := make([]string, 0, len(baseEnv)+1)
+	done := false
+	for _, e := range baseEnv {
+		if strings.HasPrefix(e, "LD_LIBRARY_PATH=") {
+			out = append(out, "LD_LIBRARY_PATH="+ldLib)
+			done = true
+			continue
+		}
+		out = append(out, e)
+	}
+	if !done {
+		out = append(out, "LD_LIBRARY_PATH="+ldLib)
+	}
+	return out
+}
+

internal/runner/blender/detect.go

@@ -0,0 +1,45 @@
+// Package blender: GPU backend detection for HIP vs NVIDIA.
+package blender
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+
+	"jiggablend/pkg/scripts"
+)
+
+// DetectGPUBackends runs a minimal Blender script to detect whether HIP (AMD) and/or
+// NVIDIA (CUDA/OptiX) devices are available. Use this to decide whether to force CPU
+// for Blender < 4.x (only force when HIP is present, since HIP has no official support pre-4).
+func DetectGPUBackends(blenderBinary, scriptDir string) (hasHIP, hasNVIDIA bool, err error) {
+	scriptPath := filepath.Join(scriptDir, "detect_gpu_backends.py")
+	if err := os.WriteFile(scriptPath, []byte(scripts.DetectGPUBackends), 0644); err != nil {
+		return false, false, fmt.Errorf("write detection script: %w", err)
+	}
+	defer os.Remove(scriptPath)
+
+	env := TarballEnv(blenderBinary, os.Environ())
+	cmd := exec.Command(blenderBinary, "-b", "--python", scriptPath)
+	cmd.Env = env
+	cmd.Dir = scriptDir
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		return false, false, fmt.Errorf("run blender detection: %w (output: %s)", err, string(out))
+	}
+
+	scanner := bufio.NewScanner(strings.NewReader(string(out)))
+	for scanner.Scan() {
+		line := strings.TrimSpace(scanner.Text())
+		switch line {
+		case "HAS_HIP":
+			hasHIP = true
+		case "HAS_NVIDIA":
+			hasNVIDIA = true
+		}
+	}
+	return hasHIP, hasNVIDIA, scanner.Err()
+}