Many macro-related bug fixes, more convenient global variable names, code style changes

[Sigmarik]

Formatted the code to match commonly used C style

Added file format extension to indicate it is written in GLSL

Renamed global specific-use seed variable to __RNG_SEED__ to indicate it is being treated as a used-inaccessible variable.
Also freed frequently used variable name seed.

Removed unused input parameter s from macro random_get_seed(). Also made it non-modifiable L-value, so syntax like random_get_seed() = 5.0 would be invalid.

Fixed many macro inconveniences that would have made them unusable in most cases.

For example, there was this beautiful transition in macro angle_difference(d,s):

#define angle_difference(d,s)   (mod(d-s+180.0, 360.0)-180.0)

So parameter s can only contain multiplication, but any exposed addition or subtraction would be inversed as soon at it is passed into macro?

Lets also take a look at the next macro

#define mean2(a,b)          ((a)+(b))/2.0

Now imagine it would be passed into the code like this:

float some_variable = 1.0 / mean2(alpha, beta);

The code above would be transformed into

float some_variable = 1.0 / ((alpha)+(beta))/2.0;

which would indeed be equivalent of

float some_variable = 0.5 / (alpha + beta);

which is not the result the user would expect from a code that originally read

float some_variable = 1.0 / mean2(alpha, beta);  // e.g. 1.0 / ((alpha + beta) / 2.0) = 2.0 / (alpha + beta)

[Sigmarik]

Oh, and, by the way, there is still some work to be done. There are three macros that will not work unless you rewrite them without parameter duplication, but at this point, I think, it would be easier and more convenient to just make them all functions already.

I marked these issued macros with TODOs.