remove deprecated `gl_FragColor` api that made compiling impossible

[iustusae]

Changed shader code to remove deprecated gl_FragColor api

The changes made to the code are intended to fix a bug that was causing the following error messages to occur:

GL version 3.3
WARNING: GLEW_ARB_debug_output is not availableERROR: could not compile GL_FRAGMENT_SHADER
ERROR: 0:8: Use of undeclared identifier 'gl_FragColor'

ERROR: failed to compile `./shaders/simple_image.frag` shader file

GL version 3.3
WARNING: GLEW_ARB_debug_output is not availableERROR: could not compile GL_FRAGMENT_SHADER
ERROR: 0:12: Use of undeclared identifier 'gl_FragColor'

ERROR: failed to compile `./shaders/simple_text.frag` shader file

GL version 3.3
WARNING: GLEW_ARB_debug_output is not availableERROR: could not compile GL_FRAGMENT_SHADER
ERROR: 0:21: Use of undeclared identifier 'gl_FragColor'

ERROR: failed to compile `./shaders/simple_epic.frag` shader file

GL version 3.3
WARNING: GLEW_ARB_debug_output is not availableERROR: could not compile GL_FRAGMENT_SHADER
ERROR: 0:21: Use of undeclared identifier 'gl_FragColor'

ERROR: failed to compile `./shaders/simple_image.frag` shader file

Summary of all the changes

The simple_color.frag, simple_epic.frag, simple_image.frag, and simple_text.frag files all underwent similar changes:

The API used to declare the output color variable gl_FragColor was deprecated, and the new API requires the use of an out variable.

from:
gl_FragColor = texture(image, out_uv); 

to:
out vec4 FragColor;
FragColor = texture(image, out_uv);

Thus, the out variables FragColor and out_color were declared in the modified files to replace the deprecated gl_FragColor.

These changes ensure that the output color variables are properly declared and can be used in the code without throwing any errors.

Additionally, in the simple_epic.frag file, the frag_uv variable was modified to use the out_uv variable instead of gl_FragCoord.xy / resolution.

from:
vec2 frag_uv = gl_FragCoord.xy / resolution;

to:
vec2 frag_uv = out_uv;

This change was made because gl_FragCoord.xy / resolution is deprecated in newer versions of OpenGL and may not be supported in future versions.

Using out_uv ensures that the code is compatible with current and future versions of OpenGL.