RFR: 8239138: StyleManager should use a BufferedInputStream

[Ambarish Rapte]

`StyleManager.calculateCheckSum()` uses a raw InputStream as the input to a `DigestInputStream` and reads one byte at a time. This is slower in performance and should be changed, either to use `BufferedInputStream` or read byte buffer of 4096 from the stream or use both.

I have tried three approaches and tested with modena.css which is ~134 KB in size.
Following are the approaches and time in milliseconds taken by the method StyleManager.calculateCheckSum(), collected from 25 readings,

1. Use BufferedInputStream and read one byte at a time [commit#1](https://github.com/openjdk/jfx/commit/6cd7d44d0ce1084c6cdb06a698c7aca127a615ef) : 
- Maximum: 53 ms,  Minimum: 27 ms, Average: 39 ms
2. Use BufferedInputStream and read buffer of 4096 at a time [commit#1+2](https://github.com/openjdk/jfx/pull/518/files/6e0c621ea62691d5402b2bca5951d1012a5b1b91)
- Maximum: 17 ms,  Minimum: 14 ms, Average: 15.58 ms
3. Use raw InputStream(current implementation) and read buffer of 4096 at a time [commit#1+2+3](https://github.com/openjdk/jfx/pull/518/files/215e1a183cfb902247f0d48685f7a901cb5fb003), which also similar to `NativeLibLoader.calculateCheckSum()` and looks faster than previous two.
- Maximum: 16 ms,  Minimum: 13 ms, Average: 14.25 ms


The time taken by `StyleManager.calculateCheckSum()` with current implementation is,
- Maximum: 61 ms,  Minimum: 38 ms, Average: 50.58 ms


Both approaches 2 & 3 show good improvement. I would prefer approach#3 as it is similar to `NativeLibLoader.calculateCheckSum()`.
However we can choose approach#2 also.
If we choose approach#3 then bug summary should be changed accordingly.

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Commit messages:
 - only buffer[4096], similar to NativeLibLoader.calculateCheckSum()
 - BufferedInputStream + buffer[4096]
 - use BufferedInputStream

Changes: https://git.openjdk.java.net/jfx/pull/518/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=518&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8239138
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/518.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/518/head:pull/518

PR: https://git.openjdk.java.net/jfx/pull/518