GPencil: Refactor of Draw Engine, Vertex Paint and all internal functions

This commit is a full refactor of the grease pencil modules including Draw Engine, Modifiers, VFX, depsgraph update, improvements in operators and conversion of Sculpt and Weight paint tools to real brushes.

Also, a huge code cleanup has been done at all levels.

Thanks to @fclem for his work and yo @pepeland and @mendio for the testing and help in the development.

Differential Revision: https://developer.blender.org/D6293

source/blender/gpu/GPU_framebuffer.h

@@ -173,6 +173,8 @@ void GPU_framebuffer_clear(GPUFrameBuffer *fb,
 #define GPU_framebuffer_clear_color_depth_stencil(fb, col, depth, stencil) \
   GPU_framebuffer_clear(fb, GPU_COLOR_BIT | GPU_DEPTH_BIT | GPU_STENCIL_BIT, col, depth, stencil)
 
+void GPU_framebuffer_multi_clear(GPUFrameBuffer *fb, const float (*clear_cols)[4]);
+
 void GPU_framebuffer_read_depth(GPUFrameBuffer *fb, int x, int y, int w, int h, float *data);
 void GPU_framebuffer_read_color(
     GPUFrameBuffer *fb, int x, int y, int w, int h, int channels, int slot, float *data);