Engine: reverted improper M_PI constant definition

The improper change to constant definition was introduced by 63de7c386e101f1f71ff47a8aa7fda5b0ffefddc by mistake. The constant cannot be declared as "float" because it looses about half in precision, causing bugs in trigonometric calculations.

Additionally suppressed few double-to-float and int-to-float warnings in 'math' unit, because the conversions are specific to current AGS math implementation.