Fix sample count limits in deferredmultisampling

The sample count was previously fixed at 8, which is not supported
on some platforms. The code now checks for the maximum supported
sample count and uses that instead.

If you would prefer the upper limit to be fixed at 8, that is
obviously trivial to change.

data/shaders/deferredmultisampling/debug.frag

@@ -8,7 +8,7 @@ layout (location = 0) in vec3 inUV;
 
 layout (location = 0) out vec4 outFragColor;
 
-#define NUM_SAMPLES 8
+layout (constant_id = 0) const int NUM_SAMPLES = 8;
 
 vec4 resolve(sampler2DMS tex, ivec2 uv)
 {