allow emulator to specify a viewport

This allows for Viewport debugging. It is configured via an environment
variable, EMULATE_READER_VIEWPORT. It works with Lua table syntax, e.g.:
EMULATE_READER_VIEWPORT="{x=10,w=550,y=5,h=790}"

frontend/device/emulator/device.lua

@@ -13,6 +13,13 @@ local Device = Generic:new{
 }
 
 function Device:init()
+    -- allows to set a viewport via environment variable
+    -- syntax is Lua table syntax, e.g. EMULATE_READER_VIEWPORT="{x=10,w=550,y=5,h=790}"
+    local viewport = os.getenv("EMULATE_READER_VIEWPORT")
+    if viewport then
+        self.viewport = require("ui/geometry"):new(loadstring("return " .. viewport)())
+    end
+
     if util.haveSDL2() then
         self.screen = require("ffi/framebuffer_SDL2_0"):new{device = self}
         self.input = require("device/input"):new{