Gpencil - Fix stroke normal from D10171

In my previous patch https://developer.blender.org/D10171 some code changing the direction the strokes normal was accidentally included. This patch reverts that back to the original normal calculation.

Reviewed By: #grease_pencil, antoniov

Differential Revision: https://developer.blender.org/D11148

Gpencil Offset Modifier - Add randomize offset options

This patch adds the Randomize options that exist in the Array modifier to the offset modifier.

Currently the patch uses
```
BLI_findindex(&gpf->strokes, gps);
```
to get the index of the current stroke for making each stroke a different seed value. This is how the noise modifier also gets the stroke seed value and it is noted there as well that this method is slow, and should be fixed in the future with another method of getting the stroke index.
Other methods were explored such as using the total number of points of the stroke, but that makes the randomize options incompatible with other modifiers before it such as Multiple Strokes, Array, Build, and Simplify.

{F9591325}

Differential Revision: https://developer.blender.org/D10171

Gpencil Offset Modifier - Add randomize offset options

This patch adds the Randomize options that exist in the Array modifier to the offset modifier.

Currently the patch uses
```
BLI_findindex(&gpf->strokes, gps);
```
to get the index of the current stroke for making each stroke a different seed value. This is how the noise modifier also gets the stroke seed value and it is noted there as well that this method is slow, and should be fixed in the future with another method of getting the stroke index.
Other methods were explored such as using the total number of points of the stroke, but that makes the randomize options incompatible with other modifiers before it such as Multiple Strokes, Array, Build, and Simplify.

{F9591325}

Differential Revision: https://developer.blender.org/D10171

Gpencil Noise - Add noise offset parameter

This patch adds a noise offset option to the grease pencil noise modifier.
It allows the user to animate the noise along the length of the stroke to create movement that is currently not possible.

It works by adding an offset to the noise table and adding the remaining floating point value to the noise table sampling.

Reviewed By: #grease_pencil

Differential Revision: https://developer.blender.org/D10021

GPencil: Noise Modifier - Move noise seed value outside of randomize subpanel

Currently the grease pencil noise modifier seed value is under the randomize subpanel.

Moved the seed value outside of this panel because it doesn't only change the noise when using the randomize option. Moving the seed value prevents it from being hidden/greyed out so the user can manually keyframe it to control the noise animation.

Reviewed By: #grease_pencil, antoniov

Differential Revision: https://developer.blender.org/D10020

GPencil Array - Add option for uniform random scaling

This patch adds the option to make the random scaling from the grease pencil array modifier uniform.
The current settings allow a separate value for each of the 3 scaling axis. The modifier also creates different seed values for each axis so there is no way to keep the random scaling uniform.
This patch creates 1 random seed value and applies it to each of the scaling axis.

Here is a demonstration of the previous behavior and the new optional behavior.
{F9485973}
{F9485981}

{F9485798}

Reviewed By: #grease_pencil, antoniov, pepeland

Differential Revision: https://developer.blender.org/D9764

Fix invert vertex group weight miscalculation for modifiers

Warp & weight vertex-group editing modifiers miscalculated vertex weight
inversion, the weights were multiplied before being subtracted from 1.

Ref D8241

Fix invert vertex group weight miscalculation for modifiers

Warp & weight vertex-group editing modifiers miscalculated vertex weight
inversion, the weights were multiplied before being subtracted from 1.

Ref D8241

Modifiers: option to preserve custom normals for subsurf & multires

Fix: Displace Modifier UV Layer UI not a pointer

The displace modifier's uv_layer property should be a pointer in the new
UI like the other modifiers use.

Differential Revision: https://developer.blender.org/D7990