Import SDL_gpu sources.

NOTE: no SCons build yet.
Enable OpenGL rendering.
Add alias for SDL_gpu/SDL_gpu/SDL_gpu.h.
Fix GPU_Init flags.
Port the load screen to use SDL_gpu.
Don't ignore CMakeLists.txt.
Commit missing CMakeLists for SDL_gpu.
Merge pull request #1 from wesnoth/master

Merge updates from wesnoth:master
add adv. preference for number of menu items displayed at once

See earlier commit: f24f6adee142c088595647d5506737c48892bf38

Forum discussion: http://forums.wesnoth.org/viewtopic.php?f=6&t=40668
change rendering of last page of wmi's, to prevent menu resize

This commit makes the pager just show enough entries from the
previous page to ensure that we have a full page. When the number
of displayed entries is large, if we don't do this then the menu
can resize dramatically which is a bit jarring.
const-correctness fixup (just for clarity)
Don't ignore txt files.
Fix SDL_gpu CMake build.

Patch provided by mordante.
add game_data::get_variable_accesss method.
move variable_info to own file

and also move game_data::temporarites_ to that file as a static
variable.

This fixes variable_info being dependent on game_data.
move partly [set_variables] code to variable_info class
add variable_info::clear method

move code from game_data

the plan is to make variable_info s member private.
add method variable_info::as_scalar_const
add variable_info::add_child method
add variable_info::as_array_throw method