Definition of several utility macros for self-contained MMX operations such as scalin...

Definition of several utility macros for self-contained MMX operations such as scaling and lerping.
Restructured the MMX blending function to use a template, being only necessary to specify the main loop, which is also used for making the runin and runout sections.
Optimization of the MMX function after remembering that the multiplication was commutative (how can somebody forget this..) resulting in less register usage. Now there is no need for generate or read from memory any constant inside the loop.

Assemblers other than the GNU assembler can choke on the output of the C preprocessor since it was necessary to add line separators ';' to the defined macros.