GPUShader: Implement workaround for gizmo drawing on sRGB framebuffer

This solution involves adding a uniform to each fragment shader that is
used by gizmo drawing and use the framebuffer state to set this uniform
accordingly.

This solution can also be carried to external shaders (addons).
A single line of code would then be enough to fix the issue.

The only trickery here is the dummy define:
`#define srgb_to_framebuffer_space(a)`
This is in order to avoid breaking other DRW shaders that use the same
fragment shader code but do not need the tranformation.

Related to T74139

Reviewed By: brecht, campbellbarton

Differential Revision: https://developer.blender.org/D7261

source/blender/gpu/GPU_shader.h

@@ -31,6 +31,7 @@ extern "C" {
 typedef struct GPUShader GPUShader;
 struct GPUTexture;
 struct GPUUniformBuffer;
+struct GPUShaderInterface;
 
 /* GPU Shader
  * - only for fragment shaders now
@@ -49,6 +50,11 @@ GPUShader *GPU_shader_create(const char *vertexcode,
                              const char *libcode,
                              const char *defines,
                              const char *shader_name);
+GPUShader *GPU_shader_create_from_python(const char *vertexcode,
+                                         const char *fragcode,
+                                         const char *geocode,
+                                         const char *libcode,
+                                         const char *defines);
 GPUShader *GPU_shader_create_ex(const char *vertexcode,
                                 const char *fragcode,
                                 const char *geocode,
@@ -83,6 +89,8 @@ int GPU_shader_get_program(GPUShader *shader);
 
 void *GPU_shader_get_interface(GPUShader *shader);
 
+void GPU_shader_set_srgb_uniform(const struct GPUShaderInterface *interface);
+
 int GPU_shader_get_uniform(GPUShader *shader, const char *name);
 int GPU_shader_get_uniform_ensure(GPUShader *shader, const char *name);
 int GPU_shader_get_builtin_uniform(GPUShader *shader, int builtin);
@@ -101,6 +109,8 @@ int GPU_shader_get_attribute(GPUShader *shader, const char *name);
 
 char *GPU_shader_get_binary(GPUShader *shader, uint *r_binary_format, int *r_binary_len);
 
+void GPU_shader_set_framebuffer_srgb_target(int use_srgb_to_linear);
+
 /* Builtin/Non-generated shaders */
 typedef enum eGPUBuiltinShader {
   /* specialized drawing */