pyxel not running on Raspberry Pi
**Describe the bug**

So I've been trying to get pyxel to work on the Raspberry Pi with pretty bad luck so far. Initially it seemed like an issue with GLFW so I ended up doing the following:

1. Compile GLFW on the Raspberry Pi with the extensions
2. Enable OpenGL in Raspbian [(Source)](https://www.raspberrypi.org/forums/viewtopic.php?t=222264#p1363568)
3. Edit `pyxel` as suggested in #23 and installed the modified version of `pyxel`
 
```
Traceback (most recent call last):
  File "game.py", line 104, in <module>
    App()
  File "game.py", line 17, in __init__
    pyxel.init(200, 200, caption='cat game')
  File "/usr/local/lib/python3.5/dist-packages/pyxel-0.9.5-py3.5.egg/pyxel/__init__.py", line 181, in init
    App(module, width, height, caption, scale, palette, fps, border_width, border_color)
  File "/usr/local/lib/python3.5/dist-packages/pyxel-0.9.5-py3.5.egg/pyxel/app.py", line 169, in __init__
    self._renderer = Renderer(width, height)
  File "/usr/local/lib/python3.5/dist-packages/pyxel-0.9.5-py3.5.egg/pyxel/renderer.py", line 48, in __init__
    self._draw_shader = GLShader(DRAWING_VERTEX_SHADER, DRAWING_FRAGMENT_SHADER)
  File "/usr/local/lib/python3.5/dist-packages/pyxel-0.9.5-py3.5.egg/pyxel/gl_wrapper.py", line 12, in __init__
    shaders.compileShader(fragment_shader, gl.GL_FRAGMENT_SHADER),validate= False
  File "/home/pi/.local/lib/python3.5/site-packages/OpenGL/GL/shaders.py", line 196, in compileProgram
    program.check_validate()
  File "/home/pi/.local/lib/python3.5/site-packages/OpenGL/GL/shaders.py", line 108, in check_validate
    glGetProgramInfoLog( self ),
RuntimeError: Validation failure (0): 
O

```


**Desktop (please complete the following information):**
 - OS: Raspbian
 - Device: Raspberry Pi
