13x7 is weird from a digital point of view. Can't you change it to something like 8X16 (128 LDRs).
Sparkfun has 16channel analogue muxer boards.
100n from each muxer input to muxer ground, pull up resisor from each muxer input to muxer VCC.
LDRs between muxer inputs and muxer ground.
Could use a Nano (8 analogue inputs) for the 8 muxer outputs (8x16) and four address lines.
Leo..