Add background border-radius to text layer

[CANEVETGASPARD]

Closes #9004

Change List

• Add backgroundBorderRadius prop to TextLayer.
• Add render test for backgroundBorderRadius prop.
• Update TextLayer api-reference documentation.

[felixpalmer]

Nice :)

[CANEVETGASPARD]

Hello,
In the last commits I improve encapsulation in fragment shader and add an if case for borderRadius. Then I use different values for borderRadius in test and fix documentation

[felixpalmer]

@CANEVETGASPARD thanks for the updates! Did you test this with sizeUnits: 'meters'? https://deck.gl/docs/api-reference/layers/text-layer#sizeunits

[CANEVETGASPARD]

I tested it this morning and it worked well

[coveralls]

coverage: 91.524% (+0.01%) from 91.512%
when pulling 980b9d4 on CANEVETGASPARD:issue-9004-border-radius-text-layer
into 4f5b90a on visgl:master.