- specify each individual particles shaders, except fullscreen_vert, to CMake.
 - the shaders file name does not need to correspond to the header file anymore.
 - the internal_format of the framebuffer is changed to GL_RGBA32F
   => we can't render unnormalized values and treat them in postpro

 - user can provide own particles shaders
 - the default particle shaders treat data as fill color
 - give a color as data for particles in main.c
 - we clear particle framebuffer to black with total transparency
 - updated points placement in triangle example

Points only send coordonates to the GPU.
Particles also send 6 other float, 2 for the speed of the particle, and 4 others (could be a color for example)
Particles are first drawn into a framebuffer, then we have a fullscreen postprocessing shader !
So it is now possible to use a lot of effects !!