Add a sample using multisample images loads and stores..

[oscarbg]

This samples show three things:
*imageLoad even not allowed on multisampled images on AMD cards.. (even on latest HD7xxx series)
*Unique ability of NV GPUs to use even imageStore with image2DMS (this is unique in all APIs (as d3d provides RWTexture2D and Texture2DMS but not RWTexture2DMS (feature for DX 12? :-) )
*Inability in GLSL of getting # of samples of attached tex (though some of your samples also suffer from lack of this having hardcoded values of 4)
Shader:

version 400 compatibility

extension GL_ARB_shader_image_load_store : enable

extension GL_ARB_shader_image_size : enable

uniform layout(rgba8) image2DMS ColorImage;
in ivec2 gl_FragCoord;
void main(void)
{
ivec2 coords = ivec2(gl_FragCoord.xy);
vec4 color=vec4(0,0,0,0);
for(int i=0; i<4; i++)
{
color += imageLoad(ColorImage, coords, i);
}
ivec2 temp=imageSize(ColorImage);
imageStore(ColorImage, coords+ivec2(1,2), gl_SampleID+1,temp.xyxy+color);
}