Fix strange d2k loading slowness

There is a strange issue that appears* when Theater calls
ISpriteFrame.Frames on the R8Reader. The R8Reader uses
IEnumerable.Cast<> which behaves slower and slower, which
makes map loading become 10+ times slower.

The changes here simply avoid the casting.

[*] This happens at least on Linux x86_64 with Mono 3.2.8.

See https://bugzilla.xamarin.com/show_bug.cgi?id=19668
Balance adjust: Return LTNK dmg. vs. wood to 75%

It seemed that the tanks were inept at hurting buildings and needed a boost. However, after playtesting, it's clear that Nod's flame tank is already exceptional at destroying buildings, and the recon bikes are quite decent at it, too. Thus, the light tank's effectiveness at killing buildings is overlapping with the flame tank (same tech level & cost), making most Nod units effective at hurting buildings. So, I think this is in order to preserve distinct roles among the vehicles.
Reduce grenadier vs. wood 75% -> 50%

Grenadiers actually do more damage than flamethrowers vs. buildings, which is counter-intuitive and not in keeping with flavour and design intent. They should be good vs. buildings, but not quite this good, especially given that they are cheaper than flamethrowers. This nerf still allows them to be effective at taking out buildings, but not quite as blindingly fast at destroying them.
Merge pull request #5286 from psydev/patch-1

Balance adjust: Return LTNK dmg. vs. wood to 75%
Merge pull request #5285 from pavlos256/d2k-theater-slow

Fix strange d2k loading slowness