AMD glitch: missing changes in f41ab375f38b

It was committed an earlier version of the patch which missed these changes.

Differential Revision: https://developer.blender.org/D4700

Fix T62792: AMD glitch when clipping region in edit mode.

The `AMD Radeon HD 7600M` compiler is usually buggy for Geometry Shaders.
In this case, indexing was causing problems in `gl_in[i].gl_ClipDistance[0]`.

Reviewers: fclem

Differential Revision: https://developer.blender.org/D4700

Fix T63340: Transform Snap Object: Simulate occlusion only when `use_occlusion_test` is true.

And cleanup.

Fix T63255: Crash when circle select performs selection outside the viewport's bounds.

Edit Mesh Selection: Clear out buffer that indicates out of bounds pixels.

This prevents the use of uninitialized buffer.
In addition, use `memset` instead of assigning in a loop.

Fix build errors

From rBd5cb425b8745

Possible fix for T62999: Crash when select in edit mode.

Apparently some drivers don't allow `glReadPixel` read out pixels of texture boundaries.
Intersect `rect` to avoid such cases.

Edit Mesh Selection: Clear buffer before using in glReadPixel.

Areas out of bound are not written.

DRW manager: Always assume that the GL_DITHER and GL_SCISSOR_TEST should be disabled if not used.

This may fix a glitch detected when you perform edited mesh selection.
See comment in rB00bdb5cdf08f

Fix T63017: selection via lasso does not work properly in X-ray.

Probably missed in rB40f8ddf8297a.