perf(draw): minor optimization on rgb565 color blending

[kisvegabor]

Description of the feature or fix

It mainly affects letter drawing and it's about 10% faster

Notes

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[C47D]

It would be good to have data to proof it's faster or some tests if the input data and output data are the same with the previous and current implementation.

[kisvegabor]

It would be good to have data to proof it's faster or some tests if the input data and output data are the same with the previous and current implementation.

We can use this when it will be ready.