lib/sdl2: intro an example for the sdl2 modules

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>

diff --git a/lib/sdl2/examples/minimal/Makefile b/lib/sdl2/examples/minimal/Makefile
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+++ b/lib/sdl2/examples/minimal/Makefile
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+all:
+       mkdir -p bin/
+       ../../../../bin/nitg -o bin/minimal src/minimal.nit

diff --git a/lib/sdl2/examples/minimal/assets/fighter.png b/lib/sdl2/examples/minimal/assets/fighter.png
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index 0000000..cf0feaa
Binary files /dev/null and b/lib/sdl2/examples/minimal/assets/fighter.png differ

diff --git a/lib/sdl2/examples/minimal/src/minimal.nit b/lib/sdl2/examples/minimal/src/minimal.nit
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+++ b/lib/sdl2/examples/minimal/src/minimal.nit
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+# This file is part of NIT ( http://www.nitlanguage.org ).
+#
+# Copyright 2014 Alexis Laferrière <alexis.laf@xymus.net>
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+# An example to test and demonstrate the `sdl2` lib with `image` and `events`
+module minimal
+
+import sdl2::all
+
+# Check for an error, then print and clear it
+#
+# Some errors are not fatal, so we ignore them at this level.
+fun check_error(loc: String)
+do
+       var error = sys.sdl.error.to_s
+       if not error.is_empty then
+               print "at {loc}: {error}"
+               sys.sdl.clear_error
+       end
+end
+
+# Init `sdl2`, including video and events
+sys.sdl.initialize((new SDLInitFlags).video)
+check_error "init"
+
+# Init `sdl2::image`
+sdl.img.initialize((new SDLImgInitFlags).png)
+check_error "img_init"
+
+# Create a window
+var window = new SDLWindow("Window title!".to_cstring, 800, 600, (new SDLWindowFlags).opengl)
+check_error "window"
+
+# Create a render, the suffested one
+var renderer = new SDLRenderer(window, -1, (new SDLRendererFlags).accelerated)
+check_error "renderer"
+
+# Load an image
+var surface = new SDLSurface.load("assets/fighter.png".to_cstring)
+check_error "surface"
+assert not surface.address_is_null
+
+# Alternative code to load a BMP image without `sdl2::image`
+#
+# var surface = new SDLSurface.load_bmp("assets/fighter.bmp".to_cstring)
+
+# Set the window icon
+window.icon = surface
+check_error "icon"
+
+# Get a texture out of that surface
+var texture = new SDLTexture.from_surface(renderer, surface)
+check_error "texture"
+
+# Allocate memory for reusable objects
+var event = new SDLEventBuffer.malloc
+var src = new SDLRect.nil
+var dst = new SDLRect(0, 0, 128, 128)
+
+# Set the colors we will be using
+var white = new SDLColor(255, 255, 255, 255)
+var green = new SDLColor(  0, 255,   0, 255)
+var spacy = new SDLColor( 25,  25,  50, 255)
+
+loop
+       # Loop over events until we get a quit event
+       while event.poll_event do
+               var higher_event = event.to_event
+               if higher_event isa SDLQuitEvent then
+                       break label out
+               else if higher_event isa SDLMouseButtonDownEvent then
+                       # Update `dst` to be centered on the latest click
+                       dst.x = higher_event.x - dst.w/2
+                       dst.y = higher_event.y - dst.h/2
+               end
+       end
+
+       # Clear the screen with a spacy color
+       renderer.draw_color = spacy
+       renderer.clear
+
+       # Draw the target box for the following `copy`
+       renderer.draw_color = green
+       renderer.draw_rect dst
+
+       # Copy a texture to the screen
+       renderer.draw_color = white
+       renderer.copy(texture, src, dst)
+
+       # Copy the back buffer to the screen
+       renderer.present
+
+       check_error "present"
+
+       33.delay
+end label out
+
+# Free all resources
+event.free
+src.free
+dst.free
+
+texture.free
+surface.free
+
+window.destroy
+sdl.img.quit
+sys.sdl.quit