[rlgl] LoadShader behaviour change

[MatthewOwens]

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Please, before submitting a new issue verify and check:

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

Issue description

When only a vertex or a fragment shader is passed to LoadShader, the shader program fails rather than falling back to the default raylib vertex/fragment shader. E.g. LoadShader(0, "shader.fs"). This is a change in behaviour introduced in 7a15966.

Environment

Provide your Platform, Operating System, OpenGL version, GPU details where you experienced the issue.
Platform: Desktop
OS: Linux
OpenGL Version: 460 (shaders are using 330)

Code Example

Example available at: https://github.com/AbyssalThistle/shader-loading-issue/https://github.com/AbyssalThistle/shader-loading-issue/blob/develop/src/main.c