YaGL: implement type conversion during data transfer

YaGL: implement type conversion during data transfer

In some cases befor transfering data to host openGL, e have to convert it to
another format. This is needed mainly for future GLES1 implementation and it needs
to cover two cases:
convert GL_FIXED data to GL_FLOAT for all transfers
convert GL_BYTE to GL_FLOAT if required (not for all transfers)

Since we cannot know for sure if we need to perform conversion when transfer occurs,
we ned to introduce new member "need_convert" in array state for this.