Rescale MR artefact property from 35-99 range to 40/80 steps

Easier to compare to other MR items, more clear in the interaction with
MR resistance line on %, gets rid of shops approximately leaking the
amount of MR, and also removes a special-cased inscription handling.

{MR+} now indicates 40 MR across the board, {MR++} stands for 80.

Almost all MR unrands had to be adjusted one way or the other. Several
of them had MR in the range 30-50 and were converted to {MR+} (40 MR).
Exceptions and other changes are listed below:

- Bear Spirit: buffed to 80 from 50
- Chilly/Flaming Death: buffed to 40 from 20
- Elemental Staff: buffed to 80 from 60
- robe of Folly: buffed to -80 from -100
- Four Winds: buffed to 120 from 100
- Jihad: loses its 20 MR
- ring of the Mage: buffed to 80 from 50
- Spriggan's Knife: buffed to 40 from 20

1 files changed, 1 insertions, 1 deletions

diff --git a/crawl-ref/source/artefact.cc b/crawl-ref/source/artefact.cc
index 4eaecb68f8..31b8d6a9f1 100644
--- a/crawl-ref/source/artefact.cc
+++ b/crawl-ref/source/artefact.cc
@@ -933,7 +933,7 @@ static void _get_randart_properties(const item_def &item,
         && one_chance_in(4 + power_level)
         && (aclass != OBJ_JEWELLERY || atype != RING_PROTECTION_FROM_MAGIC))
     {
-        proprt[ARTP_MAGIC] = 35 + random2(65);
+        proprt[ARTP_MAGIC] = one_chance_in(3) ? 2 : 1;
         power_level++;
     }