VSE: Rounded corners for timeline strips

VSE timeline strips now have rounded corners. Strip corner rounding radius is
4, 6 or 8px depending on strip height (if strip is too narrow to fit
rounding, then rounding is turned off).

This is achieved with a dedicated GPU shader for drawing most of VSE
strip widget, that it could do proper rounded corner masking.

More details and images in the PR.

Pull Request: https://projects.blender.org/blender/blender/pulls/122576

source/blender/gpu/GPU_shader_builtin.hh

@@ -75,6 +75,9 @@ enum eGPUBuiltinShader {
   /** Draw wide lines with uniform color. Has an additional clip plane parameter. */
   GPU_SHADER_3D_POLYLINE_CLIPPED_UNIFORM_COLOR,
 
+  /** Draw strip widgets in sequencer timeline. */
+  GPU_SHADER_SEQUENCER_STRIPS,
+
   /** Compute shaders to generate 2d index buffers (mainly for curve drawing). */
   GPU_SHADER_INDEXBUF_POINTS,
   GPU_SHADER_INDEXBUF_LINES,