Allow multiple declarations in for loop in a shader

[Chaosus]

This will allow the creation of more complex loop declarations like:

for(int i = 0, j = 0, k[] = {0, 1}; i < 10, j > -10; i++, j--) {
}

translated to

for (int m_i=0,m_j=0,m_k[2]=int[2](0,1);(m_i<10),(m_j>-10);m_i++,m_j--) {
}

by the shader compiler.

GLSL also allows empty statements in the for loop, so the following statement will be valid after merging this PR:

for(;;;) {
}

The only restriction I've added is for the initialization block to accept only a variable declaration, eg not allowing to write:

int i = 0;
for(i /= 10;;){
}

[akien-mga]

Thanks!