(API Change) libobs: Use single func for base effects

API removed:
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gs_effect_t *obs_get_default_effect(void);
gs_effect_t *obs_get_default_rect_effect(void);
gs_effect_t *obs_get_opaque_effect(void);
gs_effect_t *obs_get_solid_effect(void);
gs_effect_t *obs_get_bicubic_effect(void);
gs_effect_t *obs_get_lanczos_effect(void);
gs_effect_t *obs_get_bilinear_lowres_effect(void);

API added:
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gs_effect_t *obs_get_base_effect(enum obs_base_effect effect);

Summary:
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Combines multiple near-identical functions into a single function with
an enum parameter.