Rework buffer into C++ class

Cherry-picked from: jellyfin@6d3a5c7

Add 'ReusableBuffer::capacity' from [Restructure renderBlend so it can return multiple pieces; JS side not ready yet]
Cherry-picked from: jellyfin@0cc2bf8

Move private data members to the top.
Rename 'ReusableBuffer::take' -> 'ReusableBuffer::ensure_size'.

Author: JustAMan

src/SubtitleOctopus.cpp

@@ -19,50 +19,59 @@
 
 int log_level = 3;
 
-typedef struct {
+class ReusableBuffer {
+private:
     void *buffer;
     size_t size;
     size_t lessen_counter;
-} buffer_t;
 
-void* buffer_resize(buffer_t *buf, size_t new_size, int keep_content) {
-    if (buf->size >= new_size) {
-        if (buf->size >= 1.3 * new_size) {
-            // big reduction request
-            buf->lessen_counter++;
-        } else {
-            buf->lessen_counter = 0;
-        }
-        if (buf->lessen_counter < 10) {
-            // not reducing the buffer yet
-            return buf->buffer;
-        }
+public:
+    ReusableBuffer(): buffer(NULL), size(0), lessen_counter(0) {}
+
+    ~ReusableBuffer() {
+        free(buffer);
     }
 
-    void *newbuf;
-    if (keep_content) {
-        newbuf = realloc(buf->buffer, new_size);
-    } else {
-        newbuf = malloc(new_size);
+    void clear() {
+        free(buffer);
+        buffer = NULL;
+        size = 0;
+        lessen_counter = 0;
     }
-    if (!newbuf) return NULL;
 
-    if (!keep_content) free(buf->buffer);
-    buf->buffer = newbuf;
-    buf->size = new_size;
-    buf->lessen_counter = 0;
-    return buf->buffer;
-}
+    void *ensure_size(size_t new_size, bool keep_content) {
+        if (size >= new_size) {
+            if (size >= 1.3 * new_size) {
+                // big reduction request
+                lessen_counter++;
+            } else {
+                lessen_counter = 0;
+            }
+            if (lessen_counter < 10) {
+                // not reducing the buffer yet
+                return buffer;
+            }
+        }
 
-void buffer_init(buffer_t *buf) {
-    buf->buffer = NULL;
-    buf->size = 0;
-    buf->lessen_counter = 0;
-}
+        void *newbuf;
+        if (keep_content) {
+            newbuf = realloc(buffer, new_size);
+        } else {
+            newbuf = malloc(new_size);
+        }
+        if (!newbuf) return NULL;
 
-void buffer_free(buffer_t *buf) {
-    free(buf->buffer);
-}
+        if (!keep_content) free(buffer);
+        buffer = newbuf;
+        size = new_size;
+        lessen_counter = 0;
+        return buffer;
+    }
+
+    size_t capacity() const {
+        return size;
+    }
+};
 
 void msg_callback(int level, const char *fmt, va_list va, void *data) {
     if (level > log_level) // 6 for verbose
@@ -127,7 +136,7 @@ class SubtitleOctopus {
         resizeCanvas(frame_w, frame_h);
 
         reloadFonts();
-        buffer_init(&m_blend);
+        m_blend.clear();
     }
 
     /* TRACK */
@@ -173,7 +182,7 @@ class SubtitleOctopus {
         ass_free_track(track);
         ass_renderer_done(ass_renderer);
         ass_library_done(ass_library);
-        buffer_free(&m_blend);
+        m_blend.clear();
     }
     void reloadLibrary() {
         quitLibrary();
@@ -265,7 +274,7 @@ class SubtitleOctopus {
 
         // make float buffer for blending
         const size_t buffer_size = width * height * 4 * sizeof(float);
-        float* buf = (float*)buffer_resize(&m_blend, buffer_size, 0);
+        float* buf = (float*)m_blend.ensure_size(buffer_size, false);
         if (buf == NULL) {
             printf("libass: error: cannot allocate buffer for blending");
             return &m_blendResult;
@@ -345,7 +354,7 @@ class SubtitleOctopus {
     }
 
 private:
-    buffer_t m_blend;
+    ReusableBuffer m_blend;
     RenderBlendResult m_blendResult;
 };