Error checking, screen.size, and automated testing. (#116)

* More error checking with windows OpenGL
* Add function to compute screen size.
* Add functions to aid automated testing.

src/windy/common.nim

@@ -210,3 +210,10 @@ proc `[]=`*(headers: var seq[HttpHeader], key, value: string) =
       header.value = value
       return
   headers.add(HttpHeader(key: key, value: value))
+
+proc size*(screen: Screen): IVec2 =
+  ## Returns the size of the screen.
+  ivec2(
+    (screen.right - screen.left).int32,
+    (screen.bottom - screen.top).int32
+  )

src/windy/platforms/win32/platform.nim

@@ -2354,3 +2354,18 @@ proc pollEvents*() =
 
   when defined(windyUseStdHttp):
     pollHttp()
+
+proc forceMousePos*(window: Window, mousePos: IVec2) =
+  ## Forces mouse position to a place.
+  ## This is used for simulating UI tests.
+  window.state.mousePos = mousePos
+
+proc forceButtonPress*(window: Window, button: Button) =
+  ## Forces button press.
+  ## This is used for simulating UI tests.
+  window.handleButtonPress(button)
+
+proc forceButtonReleased*(window: Window, button: Button) =
+  ## Forces button release.
+  ## This is used for simulating UI tests.
+  window.handleButtonRelease(button)