Fixed -1 to implicitly unsigned char narrowing error.
Fixed -1 to implicitly unsigned char narrowing error.

C++11 does not allow narrowing a constant expression value to a type
that can't fully represent it. So when char is considered unsigned,
we can't store -1 in it. Explicitly using signed char fixes it.

Change-Id: I5c0e9fe0025659e06291655a12220ab589d5f5bd
Reviewed-on: https://swiftshader-review.googlesource.com/9630
Reviewed-by: Corentin Wallez <470c533e86f4f848c98c7e11fffc2d043494a401@google.com>
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>
Tested-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader