nir/spirv/glsl450: increase asin(x) precision

So far we were using a polynomial approximation for both asin() and acos() functions.

Unfortunately, for asin(), this polynomial does not have enough precision to satisfy Vulkan requiremenents, which defines the asin() precision as same precision as atan2(x, sqrt(1.0 - x*x)).

So let's define asin() based on the above definition, while we keep acos() using the polynomial approximation.

Closes: #1843 (closed)

Signed-off-by: Juan A. Suarez Romero jasuarez@igalia.com