Fix the vertex shader for ex_prim_shader on OSX.

Also improve the diagnostic messages when the shaders fail to compile.
Merge branch '5.1' of git://git.code.sf.net/p/alleg/allegro into 5.1
Support al_show_mouse_cursor and al_hide_mouse_cursor on Raspberry
Pi. Also remove some commented out code in Pi display driver.
Use extern "C" guards in iOS ObjC header.
Mouse buttons can stick. I noticed it when holding the mouse
and toggling fullscreen window. This partial fix makes it so
they don't stick permanently. A click will clear the stickiness.
(Mac)
Generate KEY_CHAR on Android.
MSVC requires _snprintf.
Add key constants for some Android keys, those that are
used in Xperia Play controls.
Merge branch '5.1' of git://git.code.sf.net/p/alleg/allegro into 5.1