llvmpipe: Use a simpler and faster AF implementation

This is based on the example given by the Vulkan specification: https://registry.khronos.org/vulkan/specs/1.3-extensions/html/vkspec.html#textures-texel-anisotropic-filtering

The basic idea is to compute the axis in which the uv coordinates
change the fastest in screen space (rho_y and rho_y). If rho_x is larger
than rho_y, samples are summed up along the x-Axis and the y-Axis
viceversa. The x/y offsets are mapped back into texture (u/v) space
using a linear approximation of u(x,y) and v(x,y).

This approach does not use a nested loop and the number of samples is
basically limited to max_anisotropy (+/-). The sample count of the
previous approach could explode in some situations, leading to
frametimes >1s.