add gif image reading support

this adds an included dependency for gifdec, which is in public domain.

the build process is changed to just a powershell script for now, but
that'll probably change once i rewrite this project using vulkan

README.md

@@ -62,10 +62,6 @@ options:
 
 ## Compilation
 
-On Windows, you can compile with clang (you can get it by installing [LLVM](https://releases.llvm.org/download.html)) like this:
-
-```
-clang .\osb2mp4.cpp -std=c++17 -I .\lib\ -I C:\path\to\opencv\build\include\ -L C:\path\to\opencv\build\x64\vc15\lib\ -l opencv_world451 -O3 -fopenmp -o .\osb2mp4.exe
-```
+On Windows, you can compile with clang (you can get it by installing [LLVM](https://releases.llvm.org/download.html)) by running the included `make.ps1` script. Be sure to fill in the templated variables at the top of the file.
 
 When running, make sure to have `opencv_videoio_ffmpeg451_64.dll` and `opencv_world451.dll` in the same folder as `osb2mp4.exe`.

dep/gifdec/lib/gifdec.h

@@ -0,0 +1,56 @@
+#ifndef GIFDEC_H
+#define GIFDEC_H
+
+#include <stdint.h>
+#include <sys/types.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+typedef struct gd_Palette {
+    int size;
+    uint8_t colors[0x100 * 3];
+} gd_Palette;
+
+typedef struct gd_GCE {
+    uint16_t delay;
+    uint8_t tindex;
+    uint8_t disposal;
+    int input;
+    int transparency;
+} gd_GCE;
+
+typedef struct gd_GIF {
+    int fd;
+    off_t anim_start;
+    uint16_t width, height;
+    uint16_t depth;
+    uint16_t loop_count;
+    gd_GCE gce;
+    gd_Palette *palette;
+    gd_Palette lct, gct;
+    void (*plain_text)(
+        struct gd_GIF *gif, uint16_t tx, uint16_t ty,
+        uint16_t tw, uint16_t th, uint8_t cw, uint8_t ch,
+        uint8_t fg, uint8_t bg
+    );
+    void (*comment)(struct gd_GIF *gif);
+    void (*application)(struct gd_GIF *gif, char id[8], char auth[3]);
+    uint16_t fx, fy, fw, fh;
+    uint8_t bgindex;
+    uint8_t *canvas, *frame;
+} gd_GIF;
+
+gd_GIF *gd_open_gif(const char *fname);
+int gd_get_frame(gd_GIF *gif);
+void gd_render_frame(gd_GIF *gif, uint8_t *buffer);
+int gd_is_bgcolor(gd_GIF *gif, uint8_t color[3]);
+void gd_rewind(gd_GIF *gif);
+void gd_close_gif(gd_GIF *gif);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif /* GIFDEC_H */