rdp: improved writes to 8-bit frame buffers; minimal support for read…

…ing what the CPU and RSP wrote to hidden bits on our first access to them (it’s enough to keep the two games known to depend on this happy while removing the hack of initializing hidden bits to a fixed value); don’t update memory RGB when image_read_en is clear (apparently this behavior requires elements of cycle accuracy, my previous code was only very occasionally correct here).

git-svn-id: https://svn.code.sf.net/p/angrylions-stuff/code/trunk@113 74b3176d-45c4-4b80-9022-e50c74be2ad4