Fix a logic error returning wrong values for sin(0.25) and sin(0.75)

Author: samhocevar

lpico8lib.c

@@ -65,8 +65,8 @@ static lua_Number sin_helper(lua_Number x) {
     //  - the last two bits are rounded
     // We use a lookup table of sin(x)-4x generated by PICO-8 to ensure
     // that we get the exact same results.
-    auto a = ((x.bits() & 0x4000 ? ~x : x).bits() + 2) & 0x3ffc;
-    auto ret = lua_Number::frombits(4 * a + sintable[a >> 2]);
+    auto a = ((x.bits() & 0x4000 ? ~x : x).bits() & 0x3fff) + 2;
+    auto ret = lua_Number::frombits((a >> 2 << 4) + sintable[a >> 2]);
     return x.bits() & 0x8000 ? ret : -ret;
 }