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, 5 insertions, 6 deletions

diff --git a/crawl-ref/source/shopping.cc b/crawl-ref/source/shopping.cc
index e25758d63b..37d8d4056c 100644
--- a/crawl-ref/source/shopping.cc
+++ b/crawl-ref/source/shopping.cc
@@ -867,6 +867,11 @@ int artefact_value(const item_def &item)
     else if (prop[ ARTP_COLD ] < 0)
         ret -= 10;
 
+    if (prop[ ARTP_MAGIC ] > 0)
+        ret += 4 + 4 * prop[ ARTP_MAGIC ];
+    else if (prop[ ARTP_MAGIC ] < 0)
+        ret -= 6;
+
     if (prop[ ARTP_NEGATIVE_ENERGY ] > 0)
         ret += 3 + 3 * (prop[ARTP_NEGATIVE_ENERGY] * prop[ARTP_NEGATIVE_ENERGY]);
 
@@ -878,12 +883,6 @@ int artefact_value(const item_def &item)
     if (prop[ ARTP_ELECTRICITY ])
         ret += 10;
 
-    // magic resistance is from 35-100
-    if (prop[ ARTP_MAGIC ] > 0)
-        ret += 5 + prop[ ARTP_MAGIC ] / 15;
-    else if (prop[ ARTP_MAGIC ] < 0)
-        ret -= 5;
-
     if (prop[ ARTP_EYESIGHT ])
         ret += 6;