Cache elementScreenshotSupport per browser

[alvarezguille]

Fixes #1261

[Artur-]

How is this tested?

[alvarezguille]

Tests are already failing for IE and FF in the build for all browsers

org.openqa.selenium.UnsupportedCommandException: GET /session/XXXXXXXX-XXXX-XXXX-XXXX-XXXX72480538/element/0/screenshot
Build info: version: '3.4.0', revision: 'unknown', time: 'unknown'
System info: host: 'WIN-R1JBAO1CNPS', ip: '10.100.16.13', os.name: 'Windows Server 2008 R2', os.arch: 'x86', os.version: '6.1', java.version: '1.8.0_91'
Driver info: driver.version: unknown
Command duration or timeout: 0 milliseconds
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at org.openqa.selenium.remote.ErrorHandler.createThrowable(ErrorHandler.java:214)
at org.openqa.selenium.remote.ErrorHandler.throwIfResponseFailed(ErrorHandler.java:166)
at org.openqa.selenium.remote.http.JsonHttpResponseCodec.reconstructValue(JsonHttpResponseCodec.java:40)
at org.openqa.selenium.remote.http.AbstractHttpResponseCodec.decode(AbstractHttpResponseCodec.java:80)
at org.openqa.selenium.remote.http.AbstractHttpResponseCodec.decode(AbstractHttpResponseCodec.java:44)
at org.openqa.selenium.remote.HttpCommandExecutor.execute(HttpCommandExecutor.java:158)
at org.openqa.selenium.remote.RemoteWebDriver.execute(RemoteWebDriver.java:552)
at org.openqa.selenium.remote.RemoteWebElement.execute(RemoteWebElement.java:285)
at org.openqa.selenium.remote.RemoteWebElement.getScreenshotAs(RemoteWebElement.java:392)
at com.vaadin.testbench.TestBenchElement.getScreenshotAs(TestBenchElement.java:392)
at com.vaadin.testbench.commands.ScreenshotComparator.getScreenshot(ScreenshotComparator.java:97)
at com.vaadin.testbench.commands.ScreenshotComparator.compareScreen(ScreenshotComparator.java:42)
at com.vaadin.testbench.TestBenchElement.compareScreen(TestBenchElement.java:459)
at com.vaadin.tests.elements.ElementScreenCompareIT.elementCompareScreen(ElementScreenCompareIT.java:43)