Auto merge of #6565 - dwins:issue_6452_image_padding, r=pcwalton

Fix for #6542 - <img> top-padding adds margin

I wrote this patch that makes the test from #6542 render as expected but I am not confident it is actually the right fix.  Should the padding be included in the 'ascent' metric for images, or am I just introducing a bug that happens to offset the one I'm trying to fix?

<!-- Reviewable:start -->
[<img src="https://reviewable.io/review_button.png" height=40 alt="Review on Reviewable"/>](https://reviewable.io/reviews/servo/servo/6565)
<!-- Reviewable:end -->

tests/wpt/metadata-css/css21_dev/html4/absolute-non-replaced-max-height-003.htm.ini

@@ -1,3 +0,0 @@
-[absolute-non-replaced-max-height-003.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/absolute-non-replaced-max-height-011.htm.ini

@@ -1,3 +0,0 @@
-[absolute-non-replaced-max-height-011.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/floats-040.htm.ini

@@ -1,3 +0,0 @@
-[floats-040.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/min-height-111.htm.ini

@@ -1,3 +0,0 @@
-[min-height-111.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/min-height-112.htm.ini

@@ -1,3 +0,0 @@
-[min-height-112.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/position-absolute-002.htm.ini

@@ -1,3 +0,0 @@
-[position-absolute-002.htm]
-  type: reftest
-  expected: FAIL

tests/wpt/metadata-css/css21_dev/html4/top-offset-percentage-001.htm.ini

@@ -1,3 +0,0 @@
-[top-offset-percentage-001.htm]
-  type: reftest
-  expected: FAIL