Adds Pic.openGIFDocumentFromData() (#657)

This will allow using giflib to render GIF images, which
is better than MuPDF for rendering the first frame of an
animated GIF, and will allow getting all its frames.
Also fix alpha value when combining animated GIF frames.

ffi/giflib_h.lua

@@ -75,4 +75,6 @@ static const int DISPOSE_DO_NOT       = 1;
 static const int DISPOSE_BACKGROUND   = 2;
 static const int DISPOSE_PREVIOUS     = 3;
 static const int NO_TRANSPARENT_COLOR = -1;
+typedef int (*GifInputFunc) (GifFileType *, GifByteType *, int);
+GifFileType *DGifOpen(void *, GifInputFunc, int *);
 ]]

ffi/pic.lua

@@ -166,7 +166,7 @@ function GifDocument:openPage(number)
         local palette={}
         for c=0, cmap.ColorCount-1 do
             local color = cmap.Colors[c]
-            palette[c] = BB.ColorRGB32(color.Red, color.Green, color.Blue, 0)
+            palette[c] = BB.ColorRGB32(color.Red, color.Green, color.Blue, 0xFF)
         end
 
         -- Draw current frame on our bb
@@ -221,6 +221,37 @@ function Pic.openGIFDocument(filename)
     return GifDocument:new{giffile = giffile}
 end
 
+function Pic.openGIFDocumentFromData(data, size)
+    -- Create GIF from data pointer (from https://github.com/luapower/giflib)
+    local function data_reader(r_data, r_size)
+        r_data = ffi.cast('unsigned char*', r_data)
+        return function(_, buf, sz)
+            if sz < 1 or r_size < 1 then error('eof') end
+            sz = math.min(r_size, sz)
+            ffi.copy(buf, r_data, sz)
+            r_data = r_data + sz
+            r_size = r_size - sz
+            return sz
+        end
+    end
+    local read_cb = ffi.cast('GifInputFunc', data_reader(data, size))
+    local err = ffi.new("int[1]")
+    local giffile = giflib.DGifOpen(nil, read_cb, err)
+    if giffile == nil then
+        read_cb:free()
+        error(string.format("Cannot read GIF file: %s",
+            ffi.string(giflib.GifErrorString(err[0]))))
+    end
+    if giflib.DGifSlurp(giffile) ~= giflib.GIF_OK then
+        giflib.DGifCloseFile(giffile, err)
+        read_cb:free()
+        error(string.format("Cannot parse GIF file: %s",
+            ffi.string(giflib.GifErrorString(giffile.Error))))
+    end
+    read_cb:free()
+    return GifDocument:new{giffile = giffile}
+end
+
 function Pic.openPNGDocument(filename)
     local ok, re = Png.decodeFromFile(filename)
     if not ok then error(re) end