sixel: if the client has specified a size, do not change that

We used to auto-resize images to always be a multiple of 6.

This is incorrect. The client may, for example, have specified a size
to align the image to a cell boundary, which may not be a multiple of
6.

Furthermore, in e.g. Jexer, which splits up an image into stripes,
that the next image would "overwrite" the previous one. Since the next
image was started on a cell row that the previous image had overflowed
into.

terminal.h

@@ -416,6 +416,7 @@ struct terminal {
             uint32_t *data;  /* Raw image data, in ARGB */
             int width;       /* Image width, in pixels */
             int height;      /* Image height, in pixels */
+            bool autosize;
         } image;
 
         unsigned params[5];  /* Collected parmaeters, for RASTER, COLOR_SPEC */