selfImDrawList handle on which this method operates.
p_minTop-left corner of the rectangle in absolute screen coordinates.
p_maxBottom-right corner of the rectangle in absolute screen coordinates.
col_upr_leftPacked ImU32 RGBA color (upr_left); alpha controls opacity.
col_upr_rightPacked ImU32 RGBA color (upr_right); alpha controls opacity.
col_bot_rightPacked ImU32 RGBA color (bot_right); alpha controls opacity.
col_bot_leftPacked ImU32 RGBA color (bot_left); alpha controls opacity.
Append a filled rectangle with colors interpolated from its four corners.