pixmap_scale: Don't make assumptions about inputs

Fixes #244.

Signed-off-by: Abe Wieland <[email protected]>

src/pixmap_scale.c

@@ -559,6 +559,12 @@ void pixmap_scale(enum pixmap_aa_mode scaler, const struct pixmap* src,
                   struct pixmap* dst, ssize_t x, ssize_t y, float scale,
                   bool alpha)
 {
+    // Do nothing if the scaled image won't appear on the destination pixmap
+    if (x >= (ssize_t)dst->width || (ssize_t)(x + src->width * scale) <= 0 ||
+        y >= (ssize_t)dst->height || (ssize_t)(y + src->height * scale) <= 0) {
+        return;
+    }
+
     // TODO in some cases (especially when scaling to small outputs), using
     // threads is actually slower - it may be worth implementing some better
     // heuristics here to avoid using as many (or any) threads in those cases.

src/pixmap_scale.h

@@ -26,9 +26,6 @@ extern const char* pixmap_aa_names[5];
  * @param x,y destination left top coordinates
  * @param scale scale of source pixmap
  * @param alpha flag to use alpha blending
- * Note that this function assumes -(src->width * scale) <= x < dst->width and
- * -(src->height * scale) <= y < dst->height (i.e. that at least some part of
- * the scaled image will appear on the destination)
  */
 void pixmap_scale(enum pixmap_aa_mode scaler, const struct pixmap* src,
                   struct pixmap* dst, ssize_t x, ssize_t y, float scale,