Optimized loops in JPG conversion.

[CodingFabian]

This change makes the 4 conversion loops look the same.
It optimizes access of the array length and access of the property
numComponents, which is known to be constant.

[timvandermeij]

/botio test

[pdfjsbot]

From: Bot.io (Linux)

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Received

Command cmd_test from @timvandermeij received. Current queue size: 0

Live output at: http://107.21.233.14:8877/7820d2aa7adaf5e/output.txt

[pdfjsbot]

From: Bot.io (Windows)

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Received

Command cmd_test from @timvandermeij received. Current queue size: 0

Live output at: http://107.22.172.223:8877/bd98bc90c1e03e7/output.txt

[pdfjsbot]

From: Bot.io (Windows)

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Success

Full output at http://107.22.172.223:8877/bd98bc90c1e03e7/output.txt

Total script time: 23.63 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: Passed

[pdfjsbot]

From: Bot.io (Linux)

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Success

Full output at http://107.21.233.14:8877/7820d2aa7adaf5e/output.txt

Total script time: 26.02 mins

• Font tests: Passed
• Unit tests: Passed
• Regression tests: Passed

[CodingFabian]

one remark:
I benchmarked if the for loop with i+=3 performs better or worse than the i++,i++,i++ parts.
I decided to go with i+=3 for two reasons:

1. it makes the methods look more alike, and I personally find it more readable
2. the benchmarkings did not show a clear winner. In Chrome however, the i+=3 variant was significantly faster. I cannot really explain it, but I assume that it is either that variable stores are more expensive (i++ modifies i 3 times, while i+=3 only once), or that the optimizations like it when the loop variable is not modified in the loop body.

[yurydelendik]

Thank you