fix #3066 glfwGetCursorPos parameter typing bug

1. fix `glfwGetCursorPos`(GLFW3 API):
    -  parameter type from 'i32' to 'double'.
2. add `GLFW.getMousePos`:
    - `i32` version for GLFW2.
3. fix `glfwGetMousePos`(GLFW2 API):
    - from call `GLFW.getCursorPos` to call `GLFW.getMousePos`
Merge pull request #3067 from usagi/fix-3066-glfwGetCursorPos

Fix 3066 glfw get cursor pos