new file format with versioning and layer opacity

src/canvas.h

@@ -42,6 +42,49 @@ public:
     void destroy();
 };
 
+struct PPIThumb
+{
+    int width = 128;
+    int height = 128;
+    int comp = 4;
+    bool valid() const
+    {
+        return (width == 128 && height == 128 && comp == 4);
+    }
+};
+
+struct PPIDocVersion
+{
+    int major = 0;
+    int minor = 1;
+};
+
+struct PPISoftVersion
+{
+    int major = g_version_major;
+    int minor = g_version_minor;
+    int fix = g_version_fix;
+    int build = g_version_build;
+};
+
+struct PPIHeader
+{
+    char magic[4]{ 'P', 'P', 'I', 0 };
+    PPIDocVersion doc_version;
+    PPISoftVersion soft_version;
+    PPIThumb thumb_header;
+    bool valid()
+    {
+        if (strcmp(magic, "PPI") != 0)
+            return false;
+        if (doc_version.major != 0 || doc_version.minor != 1)
+            return false;
+        if (!thumb_header.valid())
+            return false;
+        return true;
+    }
+};
+
 class Canvas
 {
 public: