Add missing textures and models from Chorus' Version

This also helps in preparing for the Nether Update (1.16). Textures were
optimised using OptiPNG by:

optipng -o7 -nx

followed by a second run with the defaults.

assets/minecraft/optifine/ctm/overlay/gravel/gravel.properties

@@ -0,0 +1,8 @@
+matchBlocks=minecraft:coal_ore minecraft:diamond_ore minecraft:emerald_ore minecraft:gold_ore minecraft:iron_ore minecraft:lapis_ore minecraft:redstone_ore minecraft:andesite minecraft:diorite minecraft:granite minecraft:clay minecraft:coarse_dirt minecraft:cobblestone minecraft:mossy_cobblestone minecraft:infested_cobblestone minecraft:dirt minecraft:grass_block minecraft:gravel minecraft:mycelium minecraft:podzol minecraft:stone minecraft:infested_stone minecraft:black_concrete_powder minecraft:blue_concrete_powder minecraft:brown_concrete_powder minecraft:cyan_concrete_powder minecraft:gray_concrete_powder minecraft:green_concrete_powder minecraft:light_blue_concrete_powder minecraft:light_gray_concrete_powder minecraft:lime_concrete_powder minecraft:magenta_concrete_powder minecraft:orange_concrete_powder minecraft:pink_concrete_powder minecraft:purple_concrete_powder minecraft:red_concrete_powder minecraft:white_concrete_powder minecraft:yellow_concrete_powder minecraft:sandstone minecraft:cut_sandstone minecraft:chiseled_sandstone minecraft:red_sandstone minecraft:chiseled_red_sandstone minecraft:cut_red_sandstone minecraft:black_terracotta minecraft:blue_terracotta minecraft:brown_terracotta minecraft:cyan_terracotta minecraft:gray_terracotta minecraft:green_terracotta minecraft:light_blue_terracotta minecraft:light_gray_terracotta minecraft:lime_terracotta minecraft:magenta_terracotta minecraft:orange_terracotta minecraft:pink_terracotta minecraft:purple_terracotta minecraft:red_terracotta minecraft:terracotta minecraft:white_terracotta minecraft:yellow_terracotta
+method=overlay
+tiles=0-16
+connect=block
+connectBlocks=gravel
+tintIndex=1
+tintBlock=gravel
+layer=cutout_mipped