Fixing rendering of primitives

[avahe-kellenberger]

Should resolve issues addressed in #220

• GPU_Pixel now renders on the center of the pixel
• GPU_Line renders from the center of pixels
• GPU_Rectangle borders are now rendered inside the rectangle instead of being centered on the bounds

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If any of these behavioral changes are unwanted, maybe we can introduce a way to allow the users to decide on the behavior. E.g. enums/booleans, new functions, etc.

Here's an illustration of the changes made to GPU_Rectangle:

• The outer black rectangle represents the outer edges of the pixels
• The inner red rectangle represents the inner edges of the pixels
• The numbers are the indices of the vertexes of the triangles used to draw the rectangle sides