Divided mouse position variables into three, one for each mouse event.

- mouseX and mouseY now exist as downMouseX, downMouseY, upMouseX, upMouseY, and moveMouseX, moveMouseY. This will hopefully simplify mouse event handling.
- Text shift bug from previous commit still exists.
Made back button highlight on mouseover, made back button change menuState to MENU_NONE on mousedown, fixed bug causing main menu text to shift after backing from MENU_ABOUT.

- context.textAlign was not explicitly defined in MENU_NONE, but was in MENU_ABOUT, so returning to MENU_NONE from MENU_ABOUT would cause it to use 'center' textAlign instead of 'left', which is the default. context.textAlign = 'left' now explicitly stated in showMenu().