fix(drawing): correct opacity handling for text and shapes (#5)

Two bugs were causing opacity to not work:

1. Swapped CA/ca parameters in registerGraphicsStateForOpacity:
   - CA (uppercase) is for stroking operations (borders)
   - ca (lowercase) is for non-stroking operations (fills, text)

2. Missing '/' prefix when setting graphics state:
   - setGraphicsState expects '/GS0' but was receiving 'GS0'

Added comprehensive opacity tests for all drawing methods including
rectangles, circles, ellipses, lines, text, paths, and images.

Author: Mythie