Avoid creation of unnecessary image bufs (at alt-text to img switch)

This is the first of a patch series for image code bugs that have
severe impact on performance.

With these testing files:

1imgA.html  =  <img src="maj00s.png" alt="img1">
2imgSA.html  = <img src="maj00s.png" alt="img1">
               <img src="maj00s.png" alt="img12">
3imgSA.html  = <img src="maj00s.png" alt="img1">
               <img src="maj00s.png" alt="img12">
               <img src="maj00s.png" alt="img123">

2imgA.html  =  <img src="maj00s.png" alt="img1">
               <img src="maj21s.png" alt="img12">
3imgA.html  =  <img src="maj00s.png" alt="img1">
               <img src="maj21s.png" alt="img12">
               <img src="c10s.png" alt="img123">

This are the results:

.------------------------------------------------------------------.
|imgbufs     |         No patch         |         Patched #1       |
|            | first time | upon reload | first time | upon reload |
|-------------------------------------------------------------------
|1imgA.html  |     2      |     1       |     1      |      1      |
|2imgSA.html |     3      |     2       |     1      |      2      |
|3imgSA.html |     4      |     3       |     1      |      3      |
|-------------------------------------------------------------------
|2imgA.html  |     4      |     2       |     2      |      2      |
|3imgA.html  |     6      |     3       |     3      |      3      |
'------------------------------------------------------------------'

1 files changed, 4 insertions, 2 deletions

diff --git a/dw/image.cc b/dw/image.cc
index 74f96e57..5df6b93e 100644
--- a/dw/image.cc
+++ b/dw/image.cc
@@ -428,8 +428,10 @@ void Image::setBuffer (core::Imgbuf *buffer, bool resize)
    if (resize)
       queueResize (0, true);
 
-   if (wasAllocated () && getContentWidth () > 0 && getContentHeight () > 0) {
-      // Only scale when both dimensions are known.
+   if (wasAllocated () && needsResize () &&
+      getContentWidth () > 0 && getContentHeight () > 0) {
+      // Don't create a new buffer for the transition from alt text to img,
+      // and only scale when both dimensions are known.
       this->buffer =
          buffer->getScaledBuf (getContentWidth (), getContentHeight ());
    } else {