Refactor ImBuf buffer access

The goal is to make it more explicit and centralized operation to
assign and steal buffer data, with proper ownership tracking.

The buffers and ownership flags are wrapped into their dedicated
structures now.

There should be no functional changes currently, it is a preparation
for allowing implicit sharing of the ImBuf buffers. Additionally, in
the future it is possible to more buffer-specific information (such
as color space) next to the buffer data itself.

The most dangerous part of this change is the change of byte buffer
data from `int*` to `uint8_t*`. In a lot of cases the byte buffer was
cast to `uchar*`, so those casts are now gone. But some code is
operating on `int*` so now there are casts in there. In practice this
should be fine, since we only support 64bit platforms, so allocations
are aligned. The real things to watch out for here is the fact that
allocation and offsetting from the byte buffer now need an explicit 4
channel multiplier.

Once everything is C++ it will be possible to simplify public
functions even further.

@@ -197,3 +197,3 @@
       /* Here we store the rect in the icon - same as before */
       if (size == bbuf->x && size == bbuf->y && xofs == 0 && yofs == 0) {
-        memcpy(iimg->rect, bbuf->rect, size * size * sizeof(int));
+        memcpy(iimg->rect, bbuf->byte_buffer.data, size * size * sizeof(int));

@@ -206,1 +205,3 @@
-              &iimg->rect[y * size], &bbuf->rect[(y + yofs) * imgsize + xofs], size * sizeof(int));
+          memcpy(&iimg->rect[y * size],
+                 &bbuf->byte_buffer.data[(y + yofs) * imgsize + xofs],
+                 size * sizeof(int));

@@ -92,3 +172,2 @@
 
-  ibuf->rect_float = nullptr;
-  ibuf->mall &= ~IB_rectfloat;
+  ibuf->flags &= ~IB_rectfloat;

@@ -553,3 +687,3 @@
 
   if (flags & IB_rect) {
-    memcpy(ibuf2->rect, ibuf1->rect, size_t(x) * y * sizeof(int));
+    memcpy(ibuf2->byte_buffer.data, ibuf1->byte_buffer.data, size_t(x) * y * sizeof(int));

@@ -154,0 +159,4 @@
+
+/* Specialization of an ownership whenever a bare pointer is provided to the ImBuf buffers
+ * assignment API. */
+typedef enum ImBufOwnership {

@@ -154,0 +160,4 @@
+/* Specialization of an ownership whenever a bare pointer is provided to the ImBuf buffers
+ * assignment API. */
+typedef enum ImBufOwnership {
+  /* The ImBug simply shares pointer with data owned by someone else, and will not perform any