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Rewrite trim_rect() for robustness.

The previous version dealt adequately with rectangles _partially_
overlapping the edge of the canvas, but doesn't correctly handle a
rectangle that's completely out of bounds in one direction. Replace
with a complete rewrite which is more easily seen to be correct. Also,
while I'm at it, add a missing condition to draw_update() so that we
don't even bother calling the Javascript half of it on any rectangle
that's been trimmed into nonexistence.

[originally from svn r9800]

--- a/emcc.c

+++ b/emcc.c

@@ -416,27 +416,30 @@

 static void trim_rect(int *x, int *y, int *w, int *h)

 {

+    int x0, x1, y0, y1;

+

     /*

      * Reduce the size of the copied rectangle to stop it going

      * outside the bounds of the canvas.

      */

-    if (*x < 0) {

-        *w += *x;

-        *x = 0;

-    }

-    if (*y < 0) {

-        *h += *y;

-        *y = 0;

-    }

-    if (*w > canvas_w - *x)

-        *w = canvas_w - *x;

-    if (*h > canvas_h - *y)

-        *h = canvas_h - *y;

-    if (*w < 0)

-        *w = 0;

-    if (*h < 0)

-        *h = 0;

+    /* Transform from x,y,w,h form into coordinates of all edges */

+    x0 = *x;

+    y0 = *y;

+    x1 = *x + *w;

+    y1 = *y + *h;

+

+    /* Clip each coordinate at both extremes of the canvas */

+    x0 = (x0 < 0 ? 0 : x0 > canvas_w ? canvas_w : x0);

+    x1 = (x1 < 0 ? 0 : x1 > canvas_w ? canvas_w : x1);

+    y0 = (y0 < 0 ? 0 : y0 > canvas_h ? canvas_h : y0);

+    y1 = (y1 < 0 ? 0 : y1 > canvas_h ? canvas_h : y1); 

+

+    /* Transform back into x,y,w,h to return */

+    *x = x0;

+    *y = y0;

+    *w = x1 - x0;

+    *h = y1 - y0;

 }

 static void js_blitter_save(void *handle, blitter *bl, int x, int y)

@@ -458,7 +461,8 @@

 static void js_draw_update(void *handle, int x, int y, int w, int h)

 {

     trim_rect(&x, &y, &w, &h);

-    js_canvas_draw_update(x, y, w, h);

+    if (w > 0 && h > 0)

+        js_canvas_draw_update(x, y, w, h);

 }

 static void js_end_draw(void *handle)