Make display of color swatches slightly robust (#463)

This reduces the risk of having invalid suffixes when converting numbers to strings.
This also supports indices that start with other than 1.
There is almost no improvement in performance.

src/display.jl

@@ -71,9 +71,10 @@ function Base.show(io::IO, mime::MIME"image/svg+xml",cs::AbstractVector{T};
         </defs>
         <rect width="$n" height="1" fill="url(#$id)" />
         """)
-    for i in 1:min(n, actual_max_swatches)
-        hexc = hex(color(cs[i]))
-        opacity = string(round(alpha(cs[i]), digits=4))
+    for (i, c) in enumerate(cs)
+        i > actual_max_swatches && break
+        hexc = hex(color(c))
+        opacity = string(round(float(alpha(c)), digits=4))
         op = replace(opacity, r"(^0(?!\.0$))|(\.0$)"=>"")
         write(io,
             """
@@ -118,11 +119,12 @@ function Base.show(io::IO, mime::MIME"image/svg+xml", cs::AbstractMatrix{T};
              shape-rendering="crispEdges" stroke="none">
         """)
 
-    for i in 1:m, j in 1:n
-        c = hex(cs[i, j])
+    i1, j1 = firstindex(cs, 1), firstindex(cs, 2)
+    for i in 0:m-1, j in 0:n-1
+        c = hex(cs[i1 + i, j1 + j])
         write(io,
             """
-            <rect width="$sw" height="$sh" x="$(j-1)" y="$(i-1)" fill="#$c" />
+            <rect width="$sw" height="$sh" x="$j" y="$i" fill="#$c" />
             """)
     end
 
@@ -171,23 +173,21 @@ function show_strokes(io::IO, mime::MIME"image/svg+xml", cs::AbstractMatrix{T};
              stroke-linecap="butt" shape-rendering="crispEdges">
         """)
 
-    for i in 1:d:m, j in 1:d:n
+    i1, j1 = firstindex(cs, 1), firstindex(cs, 2)
+    for i in 0:d:m-1, j in 0:d:n-1
         # since there is no universal way,
         # calculate the mean color in "RGB space"
-        csum = Float32[0, 0, 0]
-        u = min(n-j+1, d) # cell width
-        v = min(m-i+1, d) # cell height
-        for y in i:i+v-1, x in j:j+u-1
-            rgb = convert(RGB{Float32}, cs[y, x])
-            csum[1] += red(rgb) # avoid array allocation
-            csum[2] += green(rgb)
-            csum[3] += blue(rgb)
+        csum = RGB{Float32}(0, 0, 0)
+        u = min(n - j, d) # cell width
+        v = min(m - i, d) # cell height
+        for x in j:j+u-1, y in i:i+v-1
+            rgb = convert(RGB, cs[i1 + y, j1 + x])
+            csum = mapc((s, a) -> s + Float32(a), csum, rgb)
         end
-        csum /= (u * v)
-        c = hex(RGB{Float32}(csum[1], csum[2], csum[3]))
+        c = hex(mapc(s -> s / (u * v), csum))
         write(io,
             """
-            <path d="M$(j-1),$(i-1)h$d" stroke="#$c" />
+            <path d="M$j,$(i)h$d" stroke="#$c" />
             """)
     end