mesa: Fix glGet of ES2's GL_MAX_*_VECTORS properties.

mesa: Fix glGet of ES2's GL_MAX_*_VECTORS properties.

Previously, the get table listed all three as having custom locations,
yet find_custom_value did not have cases to handle them.

MAX_VARYING_VECTORS does not need a custom location since MaxVaryings is
already stored as float[4] (or vec4).  MaxUniformComponents is stored as
the number of floats, however, so a custom implementation that divides
by 4 is necessary.

Fixes bugs.freedesktop.org #31495.