feat: add test_image module, xorg viewer sink, and feature flag build system

Feature flags (FEATURES= make variable):
  glfw       — GLFW + OpenGL viewer (libglfw3, libglew)
  vulkan     — future Vulkan renderer
  turbojpeg  — MJPEG decode/encode (libturbojpeg)
  xorg       — XRandR geometry + screen grab (libx11, libxrandr)
  vaapi      — VA-API hardware codec (libva)
Each flag injects -DHAVE_<FEATURE> and the relevant pkg-config flags.
Headless build: make (no FEATURES set).

test_image module (src/modules/test_image/):
  Generates test frames in YUV420, YUV422, and BGRA.
  Patterns: SMPTE 75% colour bars, greyscale ramp, white/black grid.
  BT.601 limited-range RGB→YCbCr in write_pixel().

test_image_cli (dev/cli/):
  Generates a frame and writes it as a PPM file for visual verification.
  Usage: test_image_cli [--pattern bars|ramp|grid]
                        [--width N] [--height N]
                        [--format yuv420|yuv422|bgra]
                        --out FILE.ppm

xorg module (src/modules/xorg/):
  xorg.c      — full GLFW+OpenGL implementation (compiled with FEATURES=glfw)
  xorg_stub.c — no-op stub (compiled otherwise; xorg_available() returns false)
  Renderer: full-screen quad via gl_VertexID, three GL_R8 textures for YUV,
  BT.601 matrix in fragment shader, GL_BGRA texture for packed frames.
  MJPEG path: tjDecompressToYUVPlanes → planar YUV → upload (requires turbojpeg).
  push_yuv420/push_bgra/push_mjpeg all usable independently.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

include/test_image.h

@@ -0,0 +1,35 @@
+#pragma once
+
+#include <stdint.h>
+
+typedef enum Test_Pattern {
+	TEST_PATTERN_BARS,   /* SMPTE 75% colour bars */
+	TEST_PATTERN_RAMP,   /* greyscale ramp, left to right */
+	TEST_PATTERN_GRID,   /* white grid lines on black */
+} Test_Pattern;
+
+typedef enum Test_Fmt {
+	TEST_FMT_YUV420,   /* planar YUV 4:2:0 */
+	TEST_FMT_YUV422,   /* planar YUV 4:2:2 */
+	TEST_FMT_BGRA,     /* packed BGRA 8:8:8:8 */
+} Test_Fmt;
+
+typedef struct Test_Frame {
+	uint8_t  *plane[3];   /* Y/Cb/Cr; only plane[0] used for BGRA */
+	int       stride[3];  /* bytes per row for each plane          */
+	int       width;
+	int       height;
+	Test_Fmt  fmt;
+} Test_Frame;
+
+/*
+ * Allocate a Test_Frame with correctly-sized plane buffers.
+ * Width and height must be >= 2 and even.
+ * Returns NULL on allocation failure.
+ */
+Test_Frame *test_image_alloc(int width, int height, Test_Fmt fmt);
+
+/* Fill all planes of f with the given pattern. */
+void test_image_generate(Test_Frame *f, Test_Pattern pat);
+
+void test_image_free(Test_Frame *f);

include/xorg.h

@@ -0,0 +1,42 @@
+#pragma once
+
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+
+typedef struct Xorg_Viewer Xorg_Viewer;
+
+/* Returns false when compiled without HAVE_GLFW. */
+bool xorg_available(void);
+
+/*
+ * Open a viewer window at screen position (x, y) with the given size.
+ * Returns NULL if xorg is unavailable or if window/context creation fails.
+ */
+Xorg_Viewer *xorg_viewer_open(int x, int y, int width, int height,
+                               const char *title);
+
+/* Push a YUV 4:2:0 planar frame for immediate display. */
+bool xorg_viewer_push_yuv420(Xorg_Viewer *v,
+                              const uint8_t *y, const uint8_t *cb, const uint8_t *cr,
+                              int width, int height);
+
+/* Push a packed BGRA frame for immediate display. */
+bool xorg_viewer_push_bgra(Xorg_Viewer *v,
+                            const uint8_t *data, int width, int height);
+
+/*
+ * Push a MJPEG frame.  Decoded via libjpeg-turbo to planar YUV before upload.
+ * Returns false if compiled without HAVE_TURBOJPEG.
+ */
+bool xorg_viewer_push_mjpeg(Xorg_Viewer *v,
+                             const uint8_t *data, size_t size);
+
+/*
+ * Process pending window events.
+ * Returns false when the user has closed the window.
+ * Must be called from the thread that created the viewer.
+ */
+bool xorg_viewer_poll(Xorg_Viewer *v);
+
+void xorg_viewer_close(Xorg_Viewer *v);