utils.h,WEBP_ALIGN: make bitmask unsigned
utils.h,WEBP_ALIGN: make bitmask unsigned

this fixes runtime warnings with clang -fsanitize=integer when using the
WEBP_ALIGN_CST constant in WEBP_ALIGN:
implicit conversion from type 'int' of value -32 (32-bit, signed) to
type 'unsigned int' changed the value to 4294967264 (32-bit, unsigned)

Bug: b/229626362
Change-Id: I524d80e620b9869de4e6d322b7de6ffa1ec8657f

webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp,webmproject/libwebp