Add reading alpha

src/tiff/tiffread.h

@@ -111,6 +111,18 @@ struct IFD {
   u32 next_ifd;
 };
 
+typedef enum {
+  ALPHA_TYPE_UNDEFINED = 0,
+  ALPHA_TYPE_ASSOCIATED,
+  ALPHA_TYPE_UNASSOCIATED,
+} AlphaType;
+
+typedef struct tiff_alpha TiffAlpha;
+struct tiff_alpha {
+  AlphaType type;
+  u32 sample_offset;
+};
+
 typedef struct tiff_strip TiffStrip;
 struct tiff_strip {
   u32 offset;
@@ -139,6 +151,7 @@ struct tiff_image {
   u32 extra_samples;
   u32 extra_samples_count;
   bool extra_samples_offset;
+  TiffAlpha alpha;
   TiffStrip *strips;
 };