Fixes max size comparison

[Kuranes]

• left size is in Bytes, right size in MB, Fixing it so that each size is in bytes. dst_len = 242880 = 5 1024 1024 = 5 mega bytes kMaxImageSizeInMB= 64 giga bytes = 65535 mega bytes = 65535 1024 1024 = 68718428160 bytes (previous code was putting that 5mb file as dst_len = 242880 1024 1024 = 254678138880 = 242 giga bytes)

• size_t usage needs some love for 32 bits platform (wasm for example) better use explicit 64 bits for images size in bytes

[syoyo]

@Kuranes Good catch! Thanks! Merged!