Core: make fixed padding take precedence over padded widgets (fixes #42)

Prior to this commit, padding a widget meant that if there was room
after rendering the widget, the specified amount of padding would be
added. This meant that under tight layout constraints padding would
disappear before a padded widget would. This is often a desirable
outcome but it also led to unexpected behavior when adding padding to a
widget that grows greedily: fixed padding would never show up because it
was placed in a box adjacent to the widget in question, and boxes always
render greedy children before fixed ones. As a result fixed padding
would disappear under these conditions.

Instead, in the case of fixed padding, since we often intend to
*guarantee* that padding is present, all of the padding combinators have
been modified so that when the padded widget is rendered with fixed
padding in the amount V, the widget is given V fewer rows/columns when
it is rendered so that the padding always has room.

src/Brick/Widgets/Core.hs

@@ -153,7 +153,11 @@ padLeft padding p =
           Max -> (id, Greedy)
           Pad i -> (hLimit i, hSize p)
     in Widget sz (vSize p) $ do
-        result <- render p
+        c <- getContext
+        let lim = case padding of
+              Max -> c^.availWidthL
+              Pad i -> c^.availWidthL - i
+        result <- render $ hLimit lim p
         render $ (f $ vLimit (result^.imageL.to V.imageHeight) $ fill ' ') <+>
                  (Widget Fixed Fixed $ return result)
 
@@ -166,7 +170,11 @@ padRight padding p =
           Max -> (id, Greedy)
           Pad i -> (hLimit i, hSize p)
     in Widget sz (vSize p) $ do
-        result <- render p
+        c <- getContext
+        let lim = case padding of
+              Max -> c^.availWidthL
+              Pad i -> c^.availWidthL - i
+        result <- render $ hLimit lim p
         render $ (Widget Fixed Fixed $ return result) <+>
                  (f $ vLimit (result^.imageL.to V.imageHeight) $ fill ' ')
 
@@ -178,7 +186,11 @@ padTop padding p =
           Max -> (id, Greedy)
           Pad i -> (vLimit i, vSize p)
     in Widget (hSize p) sz $ do
-        result <- render p
+        c <- getContext
+        let lim = case padding of
+              Max -> c^.availHeightL
+              Pad i -> c^.availHeightL - i
+        result <- render $ vLimit lim p
         render $ (f $ hLimit (result^.imageL.to V.imageWidth) $ fill ' ') <=>
                  (Widget Fixed Fixed $ return result)
 
@@ -191,7 +203,11 @@ padBottom padding p =
           Max -> (id, Greedy)
           Pad i -> (vLimit i, vSize p)
     in Widget (hSize p) sz $ do
-        result <- render p
+        c <- getContext
+        let lim = case padding of
+              Max -> c^.availHeightL
+              Pad i -> c^.availHeightL - i
+        result <- render $ vLimit lim p
         render $ (Widget Fixed Fixed $ return result) <=>
                  (f $ hLimit (result^.imageL.to V.imageWidth) $ fill ' ')