[rshapes] Fix pixel offset issue with DrawRectangleLines

[Bigfoot71]

As requested here (#4666 (comment)), here is the fix for DrawRectangleLines.

And good news! This fix also handles scale transformations!

There was initially a small error in applying the offsets, which has been corrected.

But most importantly, strangely, the function rlGetMatrixModelview() seems to only return the first matrix of the transformation stack, which could be considered just the view matrix if we want to put it that way...

However, by calling rlGetMatrixTransform(), we seem to get the actual 'modelView' matrix, which corresponds to all the accumulated transformation matrices on the stack.

Here’s a little example:

BeginDrawing();  // rlLoadIdentity();

rlPushMatrix();
rlScalef(2, 5, 1);

Matrix mv = rlGetMatrixModelview();    //< m0 = 1 | m5 = 1
Matrix t = rlGetMatrixTransform();     //< m0 = 2 | m5 = 5

rlPopMatrix();

So, I simply replaced the call to rlGetMatrixModelview() with rlGetMatrixTransform(), fixed how the offsets are applied, and added consideration for m5 in non-uniform scales along both axes.

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Maybe rlGetMatrixModelview() is misleading in its name? It's up to you to tell me.

Also, maybe this adjustment could be applied to all other line drawing functions? I’m not sure how relevant that could be, so feel free to share your thoughts on that too.

If that’s the case, for DrawRectangleRoundedLinesEx(), it would still require some testing to be sure of what we are doing.

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Here is an example with a 10x10 square scaled using rlScalef(5, 2, 1);:

The red is at a transparency of 127, so we can see the black underneath.