[rmodels] Strange behavior when rendering flipped billboards using DrawBillboardPro

[bohonghuang]

• I tested it on latest raylib version from master branch
• I checked there is no similar issue already reported
• I checked the documentation on the wiki
• My code has no errors or misuse of raylib

Issue description

Hello! I found that when using DrawBillboardPro to render, if the height/width of the source is negative, the width/height of the size also needs to be changed to negative, otherwise the billboard will not be rendered. This may be detrimental to the API's usability and consistency. For example, when using DrawBillboardPro to render RenderTexture and flipping it upside down, both the height of source and the width of size need to be changed to negative, which is inconsistent with the behavior of DrawTexturePro. I believe the expected behavior should be able to use the source and size parameters independently to control the flipping of the billboard without interfering with each other. Any help you can provide would be greatly appreciated!

Code Example

Simply change the source variable in the billboard example of raylib to:

Rectangle source = { 0.0f, (float)bill.height, (float)bill.width, (float)-bill.height };

[raysan5]

@bohonghuang It seems like a bug, feel free to send a PR to address it.