lighting: add BUGFIX for MakeLightTable

[StephenCWills]

The i/j/k/l variables obfuscate the fact that the distance formula used for generating data in lightblock is using the x offset with the y component of the tile coordinate and vice-versa.

Mixing x and y this way causes light to move horizontally within a tile when the light source is moving vertically and vice-versa.

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To verify, if you check the places where lightblock is used, the code looks something like this.
lightblock[xoff + 8*yoff][y][x]

Compared to that, the addition and multiplication operators are swapped in lightblock[j * 8 + i][k][l], therefore it suggests the following relationships.

• i = xoff
• j = yoff
• k = y
• l = x

Subtituting these into the distance formula just a few lines up, we get:
sqrt((8 * x - yoff) * (8 * x - yoff) + (8 * y - xoff) * (8 * y - xoff))

In this formula, you can clearly see that the x and y terms are being mixed up, and swapping i and j in the equation above would swap the xoff and yoff terms in the formula. Swapping the addition and multiplication operators gives the same result, but it also matches the code in the places where lightblock is used.

See:

devilution/Source/lighting.cpp

Line 571 in 6b7135c

mult = xoff + 8 * yoff;

[StephenCWills]

I added a brief explanation in the comment to make it clearer what the bugfix fixes.